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).