natural gas hydrate A type of clathrate in which a naturally occurring gas, mainly methane, is held within a lattice of water ice. Such hydrates form under conditions of pressure and temperature found in the upper 300–2000 m of marine sediments where the water temperature is at or below 0 °C and the pressure is greater than 4 MPa, often beneath permafrost. The hydrates can block gas transmission pipes and disturbing them can cause the release of methane (a greenhouse gas). Should it prove possible to recover the methane they contain, the amount present in sediments, measured as carbon, is estimated to be up to twice that of all other recoverable and nonrecoverable fossil fuels combined.

clathrate A compound in which molecules of one substance, commonly a noble gas, are completely enclosed within the crystal structure of another substance. Typical examples are Kr and Xe encapsulated in zeolite structures, or Ar, Kr, and Xe trapped in water ice.