Gas hydrate is an ice-like material consisting of a structure of water molecules, like ice, but having an expanded crystal lattice that encages a gas molecule, generally methane. It occurs naturally within ocean sediments in a layer commonly several hundred meters thick, just below the sea floor at temperatures and pressures existing below about 500 meters water depth. Gas hydrate also is stable in conjunction with permafrost in the Arctic. Most marine gas hydrate is formed of microbially-generated gas. It binds huge amounts of methane into the sediments. Worldwide, gas hydrate is estimated to hold about 1019 grams of organic carbon in the form of methane (Kvenvolden, 1993). This represents an amount of organic carbon that is twice as much as in all other fossil fuels on earth (conventional natural gas, oil, and coal) and an amount of methane that is about 3,000 times as much as exists in the atmosphere. Gas hydrate is important primarily because it contains huge amounts of methane in a concentrated form and because it influences the physical properties of sedimentary deposits, particularly sediment strength.