Reply #14: It's not underground

[pscot]

At low temperatures methane and water form a crystaline structure that can hold huge amounts of methane. This methane ice is stable as long as temperatures in the permafrost stay low. The methane is produced by decaying plants and animals beneath the ice. There are also vast amounts of the stuff beneath the sea bed. AS the oceans warm these methane clathrates dissolve, releasing the gas.