Clathrate hydrates are a class of solids in which gas molecules occupy "cages" made up of hydrogen-bonded water molecules. These "cages" are unstable when empty, collapsing into conventional ice crystal structure, but they are stabilised by the inclusion of the gas molecule within them. Most low molecular weight gases (O2, N2, CO2, CH4, H2S, Ar, Kr, Xe etc.) will form a hydrate under some pressure-temperature conditions.
Clathrates are believed to occur in large quantities on some outer planets, binding gas at fairly high temperatures. Clathrates have also been discovered in large quantity on Earth, with hydrocarbon clathrates forming inside gas pipelines in arctic regions and giant natural methane clathrate deposits on the deep ocean floor and in permafrost regions. Deep sea deposition of CO2-clathrate to remove this greenhouse gas from the atmosphere has also been proposed.