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Overview
Methane hydrate, also known as methane clathrate or methane
ice, is a solid form of water that contains a large amount
of methane within its crystal structure.
Originally thought to occur only in the outer regions of the
Solar System where temperatures are low and water ice is
common, significant deposits of methane hydrates have been
found under sediments on the ocean floors of Earth.
Methane hydrates are common constituents of the shallow
marine geosphere, and they occur both in deep sedimentary
structures, and as outcrops on the ocean floor.
 Methane
hydrates are believed to form by migration of gas from depth
along geological faults (fractures in rock), followed by
precipitation, or crystallization, on contact of the rising
gas stream with cold sea water. Methane hydrate is also
present in deep Antarctic ice cores.
While methane hydrate is stable at a temperature of up to
around 0°C, at higher pressures it remains stable up to 18
°C.
 The
average methane hydrate composition is 1 mole of methane for
every 5.75 moles of water, though this is dependent on how
many methane molecules "fit" into the various cage
structures of the water lattice. The observed density is
around 0.9 g/cm³ (therefore, it floats in water which has a
density of 1.0 g/cm³).
One liter of methane hydrate solid would, therefore, contain
on average, 168 liters of methane gas (at standard
temperature and pressure).
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