Microwave

Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300MHz (0.3 GHz) and 300 GHz. This broad definition including both UHF and EHF (millimeter waves), and various sources use different boundaries. In all cases, microwave includes the entire SHF band (3 to 30GHz, or 10 to 1cm) at minimum, with RF engineering often putting the lower boundary at 1GHz (30cm), and the upper around 100GHz (3mm).

Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory is inaccurate. As a consequence, practical microwave technique tends to move away from the discrete resistors, capacitors, and inductors used with lower frequency radio waves. Instead, distributed circuit elements and transmission-line theory are more useful methods for design and analysis. Open-wire and coaxial transmission lines give way to waveguides, and lumped-element tuned circuits are replaced by cavity resonators or resonant lines. Effects of reflection, polarization, scattering, diffraction and atmospheric absorption usually associated with visible light are of practical significance in the study of microwave propagation. The same equations of electromagnetic theory apply at all frequencies.
While the name may suggest a micrometer wavelength, it is better understood as indicating wavelengths very much smaller than those used in radio broadcasting. The boundaries between far infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study.
Electromagnetic waves longer (lower frequency) than microwaves are called "radio waves". Electromagnetic radiation with shorter wavelengths may be called "millimeter waves", terahertz radiation or even T-rays. Definitions differ for millimeter wave band, which the IEEE defines as 110 GHz to 300 GHz.
Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is effectively opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.

Uses

Communication
Before the advent of fiber-optic transmission, most long distance telephone calls were carried via microwave point-to-point links through sites like the AT&T Long Lines. Starting in the early 1950s, frequency division multiplex was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the hop to the next site, up to 70 km away.
Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses ISM band and U-NII frequencies in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services have been used for almost a decade in many countries in the 3.5–4.0 GHz range. The FCC recently carved out spectrum for carriers that wish to offer services in this range in the U.S. — with emphasis on 3.65 GHz. Dozens of service providers across the country are securing or have already received licenses from the FCC to operate in this band. The WIMAX service offerings that can be carried on the 3.65 GHz band will give business customers another option for connectivity. Some mobile phone networks, like GSM, use the low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in the Americas and elsewhere, respectively. DVB-SH and S-DMB use 1.452 to 1.492 GHz, while proprietary/incompatible satellite radio in the U.S. uses around 2.3 GHz for DARS.
Microwave radio is used in broadcasting and telecommunication transmissions because, due to their short wavelength, highly directional antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in television news to transmit a signal from a remote location to a television station from a specially equipped van.
Most satellite communications systems operate in the C, X, Ka, or Ku bands of the microwave spectrum. These frequencies allow large bandwidth while avoiding the crowded UHF frequencies and staying below the atmospheric absorption of EHF frequencies. Satellite TV either operates in the C band for the traditional large dish fixed satellite service or Ku band for direct-broadcast satellite. Military communications run primarily over X or Ku-band links, with Ka band being used for Milstar.

Remote sensing
Radar uses microwave radiation to detect the range, speed, and other characteristics of remote objects. Development of radar was accelerated during World War II due to its great military utility. Now radar is widely used for applications such as air traffic control, weather forecasting, navigation of ships, and speed limit enforcement.
A Gunn diode oscillator and waveguide are used as a motion detector for automatic door openers (although these are being replaced by ultrasonic devices).
Most radio astronomy uses microwaves.

Power
A microwave oven passes (non-ionizing) microwave radiation (at a frequency near 2.45 GHz) through food, causing dielectric heating by absorption of energy in the water, fats and sugar contained in the food.
Microwave heating is used in industrial processes for drying and curing products.
Many semiconductor processing techniques use microwaves to generate plasma for such purposes as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD).
Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities. NASA worked in the 1970s and early 1980s to research the possibilities of using Solar power satellite (SPS) systems with large solar arrays that would beam power down to the Earth's surface via microwaves.
Less-than-lethal weaponry exists that uses millimeter waves to heat a thin layer of human skin to an intolerable temperature so as to make the targeted person move away. A two-second burst of the 95 GHz focused beam heats the skin to a temperature of 130 °F (54 °C) at a depth of 1/64th of an inch (0.4 mm). The United States Air Force and Marines are currently using this type of Active Denial System

Deep sea

The deep sea, or deep layer, is the lowest layer in the ocean, existing below the thermocline, at a depth of 1000 fathoms (1828 m) or more. Little or no light penetrates this area of the ocean, and most of its organisms rely on falling organic matter produced in the photic zone for subsistence. For this reason scientists assumed life would be sparse in the deep ocean, but virtually every probe has revealed that, on the contrary, life is abundant in the deep ocean.

From the time of Pliny until the expedition in the ship Challenger between 1872 and 1876 to prove Pliny wrong; its deep-sea dredges and trawls brought up living things from all depths that could be reached. Perhaps one day man will be more like aqua man, and roam the ocean depths with the fish creatures alike. Yet even in the twentieth century scientists continued to imagine that life at great depth was insubstantial, or somehow inconsequential. The eternal dark, the almost inconceivable pressure, and the extreme cold that exist below one thousand meters were, they thought, so forbidding as to have all but extinguished life. The reverse is in fact true....(Below 200 meters) lies the largest habitat on earth.

In 1960 the Bathyscaphe Trieste descended to the bottom of the Marianas Trench near Guam, at 35,798 feet (10,911 meters), the deepest spot on earth. If Mount Everest were submerged there, its peak would be more than a mile beneath the surface. At this great depth a small flounder-like fish was seen moving away from the bathyscaphe's spotlight. The Japanese research submersible Kaiko was the only other vessel capable of reaching this depth, but it was lost at sea in 2003.
We know more about the moon than the deepest parts of the ocean. Until the late 1970s little was known about the possibility of life on the deep ocean floor but the discovery of thriving colonies of shrimp and other organisms around hydrothermal vents changed that. Before the discovery of the undersea vents, all life was thought to be driven by the sun. But these organisms get their nutrients from the earth's mineral deposits directly. These organisms thrive in completely lightless and anaerobic environments, in highly saline water that may reach 300 °F (149 °C), drawing their sustainance from hydrogen sulfide, which is highly toxic to all terrestrial life. The revolutionary discovery that life can exist without oxygen or light significantly increases the chance of there being life elsewhere in the universe. Scientists now speculate that Europa, one of Jupiter's moons, may have conditions that could support life beneath its surface which is speculated to be a liquid ocean beneath the icy crust.

Biology

Regions below the epipelagic are divided into further zones, beginning with the mesopelagic which spans from 200 to 1000 below sea level, where a little light penetrates while still being insufficient for primary production. Below this zone the deep sea proper begins, consisting of the aphotic bathypelagic, abyssopelagic and hadopelagic. Food consists of falling organic matter known as 'marine snow' and carcasses derived from the productive zone above, and is scarce both in terms of spatial and temporal distribution.
Instead of relying on gas for their buoyancy, many species have jelly-like flesh consisting mostly of glycosaminoglycans, which has very low density. It is also common among deep water squid to combine the gelatinous tissue with a flotation chamber filled with a coelomic fluid made up of the metabolic waste product ammonium chloride, which is lighter than the surrounding water.
The midwater fish have special adaptations to cope with these conditions - they are small, usually being under 25cm; they have slow metabolisms and unspecialized diets, preferring to sit and wait for food rather than waste energy searching for it. They have elongated bodies with weak, watery muscles and skeletal structures. They often have extendable, hinged jaws with recurved teeth. Because of the sparse distribution and lack of light, finding a partner with which to breed is difficult, and many organisms are hermaphroditic.

Because light is so scarce, fish often have larger than normal, tubular eyes with only rod cells. Their upward field of vision allows them to seek out the silhouette of possible prey. Prey fish however also have adaptations to cope with predation. These adaptations are mainly concerned with reduction of silhouette, a form of camouflage. The two main methods by which this is achieved are reduction in the area of their shadow by lateral compression of the body, and counter illumination via bioluminescence. This is achieved by production of light from ventral photophores, which tend to produce such light intensity to render the underside of the fish of similar appearance to the background light. For more sensitive vision in low light, some fish have a retroreflector behind the retina. Flashlight fish have this plus photophores, which combination they use to detect eyeshine in other fish (see Tapetum lucidum).
It is important to realise that organisms in the deep sea are almost entirely reliant upon sinking living and dead organic matter which falls at approximately 100 metres per day. In addition to this, only about 1-3% of the production from the surface reaches the sea bed mostly in the form of marine snow - as mentioned above. Larger food falls, such as whale carcasses, also occur and studies have shown that these may happen more often than currently believed. There are lots of scavengers that feed primarily or entirely upon large food falls and the distance between whale carcasses is estimated to only be 8 kilometres. In addition, there are a number of filter feeders that feed upon organic particles using tentacles, such as Freyella elegans.
Marine bacteriophages play an important role in cycling nutrients in deep sea sediments. They are extremely abundant (between 5x1012 and 1x1013 phages per square metre) in sediments around the world.

Exploration

The deep sea is an environment totally inhospitable to humankind, and it should come as no surprise that it represents one of the least explored areas on Earth. Pressures even in the mesopelagic become too great for traditional exploration methods, demanding alternative approaches for deep sea research. Baited camera stations, small manned submersibles and ROVs (remotely operated vehicles) are three methods utilized to explore the ocean's depths. Because of the difficulty and cost of exploring this zone, current knowledge is limited. Pressure increases at approximately one atmosphere for every 10 metres meaning that some areas of the deep sea can reach pressures of above 1,000 atmospheres. This not only makes great depths very difficult to reach without mechanical aids, but also provides a significant difficulty when attempting to study any organisms that may live in these areas as their cell chemistry will be adapted to such vast pressures. If it were to bring them back up to laboratory conditions for study, many fish and organisms can expand or explode.

Arecibo message

The Arecibo message was beamed into space a single time (not repeated) via frequency modulated radio waves at a ceremony to mark the remodeling of the Arecibo radio telescope on 16 November 1974. It was aimed at the globular star cluster M13 some 25,000 light years away because M13 was a large and close collection of stars that was available in the sky at the time and place of the ceremony. The message consisted of 1679 binary digits, approximately 210 bytes, transmitted at a frequency of 2380 MHz and modulated by shifting the frequency by 10 Hz, with a power of 1000 kW. The "ones" and "zeroes" were transmitted by frequency shifting at the rate of 10 bits per second. The total broadcast was less than three minutes.
The cardinality of 1679 was chosen because it is a semiprime (the product of two prime numbers), to be arranged rectangularly as 73 rows by 23 columns. The alternative arrangement, 23 rows by 73 columns, produces jumbled nonsense. The message forms the image shown on the right, or its inverse, when translated into graphics characters and spaces.

Dr. Frank Drake, then at Cornell University and creator of the famous Drake equation, wrote the message, with help from Carl Sagan, among others. The message consists of seven parts that encode the following:
1. The numbers one (1) through ten (10)
2. The atomic numbers of the elements hydrogen, carbon, nitrogen, oxygen, and phosphorus, which make up deoxyribonucleic acid (DNA)
3. The formulas for the sugars and bases in the nucleotides of DNA
4. The number of nucleotides in DNA, and a graphic of the double helix structure of DNA
5. A graphic figure of a human, the dimension (physical height) of an average man, and the human population of Earth
6. A graphic of the Solar System
7. A graphic of the Arecibo radio telescope and the dimension (the physical diameter) of the transmitting antenna dish
Because it will take 25,000 years for the message to reach its intended destination of stars (and an additional 25,000 years for any reply), the Arecibo message was more a demonstration of human technological achievement than a real attempt to enter into a conversation with extraterrestrials. In fact, the stars of M13 that the message was aimed at will no longer be in that location when the message arrives. According to the Cornell News press release of November 12, 1999, the real purpose of the message was not to make contact, but to demonstrate the capabilities of newly installed equipment.

Explanation

Numbers

The numbers from 1 to 10 appear in binary format (the bottom row marks the beginning of each number).
Even assuming that recipients would recognize binary, the encoding of the numbers may not be immediately obvious due to the way they have been written. To read the first seven digits, ignore the bottom row, and read them as three binary digits from top to bottom, with the top digit being the most significant. The readings for 8, 9 and 10 are a little different, as they have been given an additional column next to the first (to the right in the image). This is probably intended to show that numbers too large to fit in a column can be written in several contiguous ones, where the contiguous columns do not have the base marker.

DNA elements

The numbers 1, 6, 7, 8 and 15 appear. These are the atomic numbers of hydrogen (H), carbon (C), nitrogen (N), oxygen (O), and phosphorus (P), the components of DNA.

Nucleotides

The nucleotides are described as sequences of the five atoms that appear on the preceding line. Each sequence represents the molecular formula of the nucleotide as incorporated into DNA (as opposed to the free form of the nucleotide).

Double helix

DNA double helix (the vertical bar represents the number of nucleotides, but the value depicted is around 4.3 billion when in fact there are about 3.2 billion base pairs in the human genome).

Humanity

The element in the center represents a human. The element on the left (in the image) indicates the average height of an adult male: 1764 mm. This corresponds to the horizontally written binary 14 multiplied by the wavelength of the message (126 mm). The element on the right depicts the size of human population in 1974, around 4.3 billion. In this case, the number is oriented horizontally rather than vertically, with the least-significant-digit marker to the upper left in the image.

Planets

The solar system, showing the Sun and the planets in the order of their position from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. (Pluto has since been reclassified as a dwarf planet by the International Astronomical Union, but it was still considered a planet at the time the message was transmitted.)
The Earth is the third planet from the Sun - its graphic is shifted up to identify it as the planet from which the signal was sent. The human figure is shown "standing on" the Earth graphic.
In addition to showing position, the graphic provides a general, not-to-scale size reference of each planet and the Sun.

Telescope

The last part represents the Arecibo radio telescope with its diameter (2430 multiplied by the wavelength gives 306.18 m). In this case, the number is oriented horizontally, with the least-significant-digit marker to the lower right in the image.

Gunung Mulu National Park

Gunung Mulu National Park near Miri, Sarawak, Malaysian Borneo, is a UNESCO World Heritage Site that encompasses incredible caves and karst formations in a mountainous equatorial rainforest setting. The park is famous for its caves and the expeditions that have been mounted to explore them and their surrounding rainforest, most notably the Royal Geographical Society Expedition of 1977 - 1978, which saw over 100 scientists in the field for 15 months.
The national park is named after Mount Mulu, the second highest mountain in Sarawak. Mount Mulu is a sandstone and shale mountain. It is the second highest mountain in the state of Sarawak, after Mount Murud. It is located within the boundaries of Gunung Mulu National Park, which is named after it.
Mount Mulu is 2376 m.a.s.l. The Mulu Park provides for guided hikes up the mountain. The usual schedule is a 4D/3N hike. There are forest huts along the route at Camp 1, Camp 3 and Camp 4. These forest huts provide shelter, cooking facilities (stove, cooking utensils, cutlery), toilets, and water (collected rainwater).
The elevation and distance of the Camps For Mulu Park HQ are approximately: Camp 1: 200 masl, 5.5km Camp 3: 1300 masl, 12 km Camp 4: 1800 masl, 18.5km
There is no Camp 2. Each of the camps is near a helipad for emergency evacuations and also for supplies.
The trail to Gng Mulu is clear and well-marked with red and white markers, and goes through a variety of ecosystems, from lowland dipterocarp forest to montane vegetations.

Geology and landforms

Gunung Mulu National Park is famous for its limestone karst formations. Features include enormous caves, vast cave networks, rock pinnacles, cliffs and gorges.
Gunung Mulu National Park has the largest known natural chamber or room - Sarawak chamber, found in Gua Nasib Bagus. It is 2,300 feet (700 m) long, 1,300 feet (396 m) wide and at least 230 feet (70 m) high; according to the guides it is big enough to fit St. Peter's Basilica or several jumbojets inside. Other notable caves are Benarat Cavern, Deer Cave, Wind Cave, and Clearwater Cave, which exposes parts of a long underground river going through the park.
In April 2009, a newly-discovered cave in Vietnam overtook Deer Cave as the largest cave passage. The Son Doong Cave in Phong Nha-Ke Bang National Park in Quang Binh Province, Vietnam was found by British cave scientists of the British Cave Research Association, it is now regarded as the largest cave passage in the world. The biggest passage of Son Doong is over five kilometers in length, 200 meters high and 150 meters wide.
Mulu's limestones belong to the Melinau Formation and their age is between 17 and 40 million years (Late Eocene to Early Miocene).
Stratigraphically below the limestones, and forming the highest peaks in the south east sector of the Park including Gunung Mulu, lies the Mulu Formation (shales and sandstones). The age of these rocks is between 40 and 90 million years (Late Cretaceous to Late Eocene).

Fauna

Eight species of hornbill have been spotted in Mulu including the Rhinoceros Hornbill Buceros rhinoceros which features on Sarawak state emblem, the White-crowned Hornbill Berenicornis/Aceros comatus and the Helmeted Hornbill Buceros vigil with its large solid casque (bill).
Twenty seven species of bat have been recorded in Mulu. Deer Cave in the southern limestone hills of the park is home to an enormous colony of Wrinkle-lipped bats Tadarida plicata. The bats exit the cave almost every evening in search of food in a spectacular exodus. A huge mound of guano in the cave is evidence of the size of the bat colony that roosts in the cave's high ceilings.
Mulu's mammals also include the Bearded pig Sus barbatus, the moonrat Echinosorex gymnurus, shrews, the Bornean Tarsier Tarsius bancanus, the long-tailed Macaque Macaca fascicularis, gibbons, squirrels, and three types of deer including the small barking deer and mouse deer. The small Malaysian sun bear Helarctos malayanus, which is the only bear known in South-East Asia, has also been identified in Gunung Mulu National Park.

Flora

Gunung Mulu National Park contains a large number of plant species, including flowering plants, trees, and fungi. Geology, soil types and topography have given rise to a rich tapestry of plant zones and types. On Gunung Mulu itself these include lowland mixed dipterocarp forest, lower montane forest, mossy or upper montane forest and summit zone vegetation on the highest peaks. On the limestones there is lowland limestone forest, lower and upper montane limestone forest. Other plant communities dominate the alluvial plains, including kerangas (tropical heath forest) and peatswamp forest.

Access

Mulu is a very inaccessible area; the only practical way of getting to and from it is by air, mainly from Miri airport. It is possible to travel to the area by riverboat, but it requires a chartered long boat for the last part - and the whole trip by river would take around 12 hours to complete from Miri, while the flight takes only 30 minutes. Prior to the opening of the airport, and the opening of a helipad in 1991, this was the only way to reach the national park.
Excursions to Mulu continues to retain the sense of adventure associated with its original exploration through the provision of adventure caving and other adventure activities. The primary focus however has shifted to the promotion of an awareness of the significance of the park and its environment through the provision of ecotourism activities that foster understanding and appreciation of the parks values. Accommodation is available at the five star luxury resort Royal Mulu Resort, the tropical-style boutique hotel The Matumau Lodge, or there are cheaper lodgings across the river.

Sea cave

A sea cave, also known as a littoral cave, is a type of cave formed primarily by the wave action of the sea. The primary process involved is erosion. Sea caves are found throughout the world, actively forming along present coastlines and as relict sea caves on former coastlines. In places like Thailand's Phang Nga Bay, solutional caves have been flooded by the rising sea and are now subject to littoral erosion.
Some of the best-known sea caves are European. Fingal's Cave, on the Scottish island of Staffa, is a spacious cave some 70 m long, formed in columnar basalt. The Blue Grotto of Capri, although smaller, is famous for the apparent luminescent quality of its water, imparted by light passing through openings underwater. The Romans built a stairway in its rear and a now-collapsed tunnel to the surface. The Greek islands are also noted for the variety and beauty of their sea caves. Numerous sea caves have been surveyed in England, Scotland, and in France, particularly on the Normandy coast. The largest sea caves are found along the west coast of the United States and in the Hawaiian islands.

Formation

Littoral caves may be found in a wide variety of host rocks, ranging from sedimentary to metamorphic to igneous, but caves in the latter tend to be larger due to the greater strength of the host rock.
In order to form a sea cave, the host rock must first contain a weak zone. In metamorphic or igneous rock, this is typically either a fault as in the caves of the Channel Islands of California, or a dike as in the large sea caves of Kauai, Hawaii’s Na Pali Coast. In sedimentary rocks, this may be a bedding-plane parting or a contact between layers of different hardness. The latter may also occur in igneous rocks, such as in the caves on Santa Cruz Island, California, where waves have attacked the contact between the andesitic basalt and the agglomerate.
The driving force in littoral cave development is wave action. Erosion is ongoing anywhere that waves batter rocky coasts, but where sea cliffs contain zones of weakness, rock is removed at a greater rate along these zones. As the sea reaches into the fissures thus formed, they begin to widen and deepen due to the tremendous force exerted within a confined space, not only by direct action of the surf and any rock particles that it bears, but also by compression of air within. Blowholes (partially submerged caves that eject large sprays of sea water as waves retreat and allow rapid re-expansion of air compressed within), attest to this process. Adding to the hydraulic power of the waves is the abrasive force of suspended sand and rock. Most sea-cave walls are irregular and chunky, reflecting an erosional process where the rock is fractured piece by piece. However, some caves have portions where the walls are rounded and smoothed, typically floored with cobbles, and result from the swirling motion of these cobbles in the surf zone.
True littoral caves should not be confused with inland caves that have been intersected and revealed when a sea cliff line is eroded back, or with dissolutional voids formed in the littoral zone on tropical islands (see Speleogenesis: Coastal and Oceanic Settings). In some regions, such as Halong Bay, Vietnam, caves in carbonate rocks are found in littoral zones but were formed by dissolution.
Rainwater may also influence sea-cave formation. Carbonic and organic acids leached from the soil may assist in weakening rock within fissures. As in solutional caves, small speleothems may develop in sea caves.
Sea cave chambers sometimes collapse leaving a “littoral sinkhole”. These may be quite large, such as Oregon’s Devil’s Punchbowl or the Queen’s Bath on the Na Pali coast. Small peninsulas or headlands often have caves that cut completely through them, since they are subject to attack from both sides, and the collapse of a sea cave tunnel can leave a free-standing “sea stack” along the coast. The Californian island of Anacapa is thought to have been split into three islets by such a process.
Life within sea caves may assist in their enlargement as well. For example, sea urchins drill their way into the rock, and over successive generations may remove considerable bedrock from the floors and lower walls. You might not find a big variety of fishes.

Factors influencing size

Most sea caves are small in relation to other cave types. A current compilation of sea-cave surveys Long sea caves of the world shows three over 300 meters, 15 over 200 meters, and 85 over 100 meters in length. In Norway, several apparently relict sea caves exceed 300 meters in length. There is no doubt that many other large sea caves exist but have not been investigated due to their remote locations and/or hostile sea conditions.
Several factors contribute to the development of relatively large sea caves. The nature of the zone of weakness itself is surely a factor, although difficult to quantify. A more readily observed factor is the situation of the cave’s entrance relative to prevailing sea conditions. At Santa Cruz Island, the largest caves face into the prevailing northwest swell conditions—a factor which also makes them more difficult to survey. Caves in well-protected bays sheltered from prevailing seas and winds tend to be smaller, as are caves in areas where the seas tend to be calmer.
The type of host rock is important as well. All of the largest sea caves are in basalt, a strong host rock compared to sedimentary rock. Basaltic caves can penetrate far into cliffs where most of the surface erodes relatively slowly. In weaker rock, erosion along a weaker zone may not greatly outstrip that of the cliff face.
Time is another factor. The active littoral zone changes throughout geological time by an interplay between sea-level change and regional uplift. Recurrent ice ages during the Pleistocene have changed sea levels within a vertical range of some 200 meters. Significant sea caves have formed in the California Channel Islands that are now totally submerged by the rise in sea levels over the last 12 000 years. In regions of steady uplift, continual littoral erosion may produce sea caves of great height — Painted Cave is almost 40 m high at its entrance.
Finally, caves that are larger tend to be more complex. By far the majority of sea caves consist of a single passage or chamber. Those formed on faults tend to have canyon-like or angled passages that are very straight. In Seal Canyon Cave on Santa Cruz Island, entrance light is still visible from the back of the cave 189 m from the entrance. By contrast, caves formed along horizontal bedding planes tend to be wider with lower ceiling heights. In some areas, sea caves may have dry upper levels, lifted above the active littoral zone by regional uplift.
Sea caves can prove surprisingly complex where numerous zones of weakness—often faults—converge. In Catacombs Cave on Anacapa Island (California), at least six faults intersect. In several caves of the Californian Channel Islands, long fissure passages open up into large chambers beyond. This is invariably associated with intersection of a second fault oriented almost perpendicularly to that along the entrance passage.

Nuclear fuel

Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. By far the most common type of nuclear fuel is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor; nuclear fuel in a nuclear fuel cycle can refer to the material or to physical objects (for example fuel bundles composed of fuel rods) composed of the fuel material, perhaps mixed with structural, neutron moderating, or neutron reflecting materials. The most common fissile nuclear fuels are 235U and 239Pu, and the actions of mining, refining, purifying, using, and ultimately disposing of these elements together make up the nuclear fuel cycle, which is important for its relevance to nuclear power generation and nuclear weapons.
Not all nuclear fuels are used in fission chain reactions. For example, 238Pu and some other elements are used to produce small amounts of nuclear power by radioactive decay in radiothermal generators, and other atomic batteries. Light isotopes such as 3H (tritium) are used as fuel for nuclear fusion. If one looks at binding energy of specific isotopes, there can be an energy gain from fusing most elements with a lower atomic number than iron, and fissioning isotopes with a higher atomic number than iron.

Common physical forms of nuclear fuel

For use as nuclear fuel, enriched UF6 is converted into uranium dioxide (UO2) powder that is then processed into pellet form. The pellets are then fired in a high-temperature, sintering furnace to create hard, ceramic pellets of enriched uranium. The cylindrical pellets then undergo a grinding process to achieve a uniform pellet size. The pellets are stacked, according to each nuclear core's design specifications, into tubes of corrosion-resistant metal alloy. The tubes are sealed to contain the fuel pellets: these tubes are called fuel rods. The finished fuel rods are grouped in special fuel assemblies that are then used to build up the nuclear fuel core of a power reactor.
The metal used for the tubes depends on the design of the reactor - stainless steel was used in the past, but most reactors now use a zirconium alloy, which in addition to being highly corrosion resistant, has low neutron absorption. For the most common types of reactors (BWRs and PWRs) the tubes are assembled into bundles with the tubes spaced precise distances apart. These bundles are then given a unique identification number, which enables them to be tracked from manufacture through use and into disposal.

PWR fuel

Pressurized water reactor (PWR) fuel consists of cylindrical rods put into bundles. A uranium oxide ceramic is formed into pellets and inserted into Zircaloy tubes that are bundled together. The Zircaloy tubes are about 1 cm in diameter, and the fuel cladding gap is filled with helium gas to improve the conduction of heat from the fuel to the cladding. There are about 179-264 fuel rods per fuel bundle and about 121 to 193 fuel bundles are loaded into a reactor core. Generally, the fuel bundles consist of fuel rods bundled 14x14 to 17x17. PWR fuel bundles are about 4 meters in length. In PWR fuel bundles, control rods are inserted through the top directly into the fuel bundle. The fuel bundles usually are enriched several percent in 235U. The uranium oxide is dried before inserting into the tubes to try to eliminate moisture in the ceramic fuel that can lead to corrosion and hydrogen embrittlement. The Zircaloy tubes are pressurized with helium to try to minimize pellet cladding interaction (PCI) which can lead to fuel rod failure over long periods.

BWR fuel
In boiling water reactors (BWR), the fuel is similar to PWR fuel except that the bundles are "canned"; that is, there is a thin tube surrounding each bundle. This is primarily done to prevent local density variations from effecting neutronics and thermal hydraulics of the nuclear core on a global scale. In modern BWR fuel bundles, there are either 91, 92, or 96 fuel rods per assembly depending on the manufacturer. A range between 368 assemblies for the smallest and 800 assemblies for the largest U.S. BWR forms the reactor core. Each BWR fuel rod is back filled with helium to a pressure of about three atmospheres (300 kPa).

CANDU fuel

CANDU fuel bundles are about a half meter in length and 10 cm in diameter. They consist of sintered (UO2) pellets in zirconium alloy tubes, welded to zirconium alloy end plates. Each bundle is roughly 20 kg, and a typical core loading is on the order of 4500-6500 bundles, depending on the design. Modern types typically have 37 identical fuel pins radially arranged about the long axis of the bundle, but in the past several different configurations and numbers of pins have been used. The CANFLEX bundle has 43 fuel elements, with two element sizes. It is also about 10 cm (four inches) in diameter, 0.5 m (20 inches) long and weighs about 20 kg (44 lbs) and replaces the 37-pin standard bundle. It has been designed specifically to increase fuel performance by utilizing two different pin diameters. Current CANDU designs do not need enriched uranium to achieve criticality (due to their more efficient heavy water moderator), however, some newer concepts call for low enrichment to help reduce the size of the reactors.

Spent nuclear fuel

Used nuclear fuel is a complex mixture of the fission products, uranium, plutonium and the transplutonium metals. In fuel which has been used at high temperature in power reactors it is common for the fuel to be heterogeneous; often the fuel will contain nanoparticles of platinum group metals such as palladium. Also the fuel may well have cracked, swelled and been used close to its melting point. Despite the fact that the used fuel can be cracked, it is very insoluble in water, and is able to retain the vast majority of the actinides and fission products within the uranium dioxide crystal lattice.

Islamic Missions

Dawah means to "invite" (in Arabic, literally "calling") to Islam, estimated to be the second largest religion next to Christianity. From the 7th century it spread rapidly from the Arabian Peninsula to the rest of the world through the initial Arabic conquests, and subsequently with traders and explorers after the death of the Prophet Muhammad SAW. Initially, the spread of Islam came through the dawah efforts of Muhammad and those who followed him. After his death in 632 CE, much of the expansion of the empire came through conquest, such as that of North Africa and later Spain (Al-Andalus), and the Islamic conquest of Persia putting an end to the Sassanid Empire and spreading the reach of Islam to as far East as Khorasan, which would later become the cradle of Islamic civilization during the Islamic Golden Age and a stepping-stone towards the introduction of Islam to the Turkic tribes living in and bordering the area. The missionary movements peaked during the Islamic Golden Age, with the expansion of foreign trade routes, primarily into the Indo-Pacific and as far South as the isle of Zanzibar and the South-Eastern shores of Africa. With the coming about of the tradition of Sufism, Islamic missionary activities have increased considerably. The mystical nature of the tradition had an all-encompassing aspect, a property many societies in Asia could relate to. Later, with the conquest of Anatolia by the Seljuk Turks, missionaries would find easier passage to the lands then formerly belonging to the Byzantine Empire. In the earlier stages of the Ottoman Empire, a Turkic form of Shamanism was still widely practiced in Anatolia, which soon started to give in to the mysticism offered by Sufism. The teachings of Jalal ad-Din Muhammad Rumi, who migrated from Khorasan to Anatolia, are good examples to the mystical aspect of Sufism. During the Ottoman presence in the Balkans, missionary movements were also taken up by people from aristocratic families hailing from the region, who had been educated in Constantinople or any other major city within the Empire, in famed madrassahs and kulliyes. Most of the time, such individuals were sent back to the place of their origin, being appointed important positions in the local governing body. This approach often resulted in the building of mosques and local kulliyes for future generations to benefit from, as well as spreading the teachings of Islam. The spread of Islam towards Central and West Africa has been prominent but slow, until the early 19th century. Previously, the only connection was through Transsaharan trade, of which the Mali Empire, consisting predominantly of African and Berber tribes, stands as a strong proof of the early Islamization of the Sub-Saharan region. The gateways prominently expanded to include the aforementioned trade routes through the Eastern shores of the African continent. With the European colonization of Africa, missionaries were almost in competition with the European Christian missionaries operating in the colonies. The Muslim population of the US has increased greatly in the last one hundred years, with much of the growth driven by widespread conversion. Up to one-third of American Muslims are African Americans who have converted to Islam during the last seventy years. Conversion to Islam in prisons, and in large urban areas has also contributed to its growth over the years.

  

Missionary Activity in North America

The Muslim population of the US has increased greatly in the last one hundred years, with much of the growth driven by widespread conversion. This conversion phenomenon can be sub-divided into several separate missionary efforts that have sprung up primarily over the past sixty years.

Nation of Islam Black superamist group Nation of Islam's efforts to recruit members to its fold would be the earliest example of Islamic missionary activity in the United States. While considered a heretic branch of Islam, former Nation of Islam converts have gone on to become major figures in the mainstream Islamic presence in North America. Malcolm X, Muhammad Ali and founder Elijah Muhammad's own son, Warith Deen Mohammed being prime examples.

Mosque Building Phenomenon The arrival of a new class of educated professionals and higher education seeking foreign student Muslim immigrants beginning in the 1970s heralded the beginning of a major mosque building phenomenon all across the North American landscape. As communities grew over the next two decades, with more immigrants from the Muslim world and with first generation children of the first wave of immigrants, small rooms serving as community centers grew into full fledged mosques. A common occurrence being the purchase of abandoned Churches and conversion into mosques. With the development of mosques and more stable Muslim communities, missionary activity has followed with mosques developing "dawah programs" to preach to local neighbors in their midst.

Interaction with Immigrants A major form of unplanned missionary activity has occurred simply due to the interaction between the local non-Muslim populace and the new wave of Muslim immigrants, at work, in schools, as neighbors and at universities. The flow of information of ideas has resulted in many a converts to the relatively new religion.

Missionary Work in Prison Systems Main article: Conversion to Islam in prisons A more recent missionary front has been the US Prison System, where encouragement of religious study has opened an avenue for Muslims to provide their own religion. There is an increasing trend towards hiring of full-time Muslim Chaplins to cater to increasing populations of Muslim prisoners and in large urban areas.

Saudi-Financed Missionary Work With the burgeoning Muslim population in North America by the late 1980s, numerous missionary outlets saw an opportunity to receive financing for their work from various Saudi-based religious foundations. This phenomenon, which flourished for much of the decade of the 1990s, came to an abrupt end following the events of the September 11 attacks. Some of the works undertaken in this time included: a) Mass distribution of A Brief Illustrated Guide to Understanding Islam (ISBN 9960-34-011-2) a high quality color booklet widely available at missionary outlets. b) Mass distribution of the complete Yusuf Ali translation The Meaning of the Holy Qur'an. Tens of thousands of the US Amana Publications edition (ISBN 978-1590080252) were available for free at missionary outlets across North America during the 1990s. These were printed under the auspices of the Iqraa Charitable Society of Jeddah, Saudi Arabia.