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, and occur offshore or on land, often beneath permafrost. The hydrates can block gas transmission pipes and disturbing them can cause the release of methane (a greenhouse gas). Natural gas hydrates are likely to be developed as a source of recoverable methane and the amount present in sediments, measured as carbon, is estimated to be more than twice that of all other recoverable and non-recoverable fossil fuels combined.