Imagine a mixture of ice and methane.  Such a mixture is possible because the structure of ice is open enough to accommodate the small methane molecules.   This mixture, called methane hydrate, will actually burn, and when the ice melts, the methane gas is released and has the same properties as methane from any other source.   Methane hydrate has been found off the coasts of North and South Carolina and Oregon.  While it is somewhat unusual to find methane hydrate so close to land, it is a fairly common substance deep in the ocean, where high pressures and low temperatures sometimes form ice that contains trapped methane gas.  The U.S. Geological Service estimates the total amount of carbon trapped in methane hydrate on the ocean floors to be over twice the amount of all the carbon found in all the known fossil fuel deposits, including petroleum, coal, and natural gas.

The deposits off the coast of the Carolinas are approximately the size of Rhode Island and contain about 1300 trillion cubic feet of methane.  That's about 65 times the current annual U.S. consumption of natural gas.   Most of this methane is thought to have been formed by the decomposition of marine sediments by bacteria, but some may be from deep sediments that have existed on the ocean floor for eons.  When released from the sediments by geologic changes and increases in temperature, the methane is then trapped in the cage-like structures formed by water molecules as they hydrogen-bond together to form ice.  Large deposits of methane also act as a cement, holding entire layers of sediment together.  These layers can also trap large deposits of regular gaseous methane.

It is generally believed that the methane hydrate deposits can someday be tapped to provide a tremendous amount of usable methane for the world's population.  But scientists are just beginning to appreciate all the factors involved in getting at these deposits.  Drilling into them will probably release some of the methane gas trapped between sediment layers.  This, in turn, will lower the pressure on the layers of methane hydrate sediment below and probably release more methane gas, which can then be collected and used for energy.  But if the entire methane hydrate deposit over a large area were upset, huge quantities of methane would be released into the ocean water, where it is not very soluble, and it would then escape into the atmosphere where it would act as a greenhouse gas.  Because methane is about 10 times as effective as a greenhouse gas compared to carbon dioxide, large amounts of methane released into the atmosphere could affect global temperatures over a short period of time.

For now, careful drilling is being planned off the Carolina coastline, and methane hydrate will probably produce usable methane in the very near future.

Moore/Stanitsky/Wood/Kotz:  The Chemical World: Concepts and Applications,  2/e,  p. 460