Methane clathrate hydrate is a form of water ice 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 common, extremely large deposits of methane clathrates have been found under sediments on the ocean floors of Earth. Methane clathrates 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, followed by precipitation, or crystallization, on contact of the rising gas stream with cold sea water.
The combination of low temperature and high pressure found at the bottom of Earth's oceans makes methane clathrates very stable. It is thought that as much as 20 times the current known reserves of natural gas may be contained within ocean-floor clathrate deposits, representing a potentially important future source of fossil fuel. Methane clathrates remain stable at temperatures up to 18 °C. The average methane clathrate 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. One liter of methane clathrate solid would therefore contain, on average, 168 liters of methane gas (at STP).
Sudden release of methane clathrate has been hypothesized as a cause of past climate changes, because methane is a greenhouse gas.