Sally-

Firstly, I apparently should spell-check my posts more carefully. I meant clathrate, not cladrate.

A clathrate is essentially a lattice structure of molecules that contains a second type of molecule. Methane clathrates are lattices of water ice with large amounts of methane gas trapped inside them.

Basically the gas can bubble out of the earth, and crystallises when it makes contact with cold water. You find them at the bottom of deep lakes and the ocean, and in permafrost.

The concern with clathrates such as these is that, as the water ice caging the methane melts, then the gas can be released. A typical clathrate contains a disproportionately large amount of methane gas, so obviously substantial releases would have significant implications.

Interestingly though, such clathrates could be a source of natural gas for fuel, but the difficulty is finding them concentrated enough for it to be commercially viable.