|Methane is a gas. Its molecules consist of one carbon atom
at the center of a triangular-pyramidal structure (tetrahedral structure)
with hydrogen atoms at the four vertices. Methane is lighter than
air and tends to rise. into the atmosphere.
However, at the low temperatures (just above the freezing point of water) and extremely high pressure at the bottom of the ocean, methane can combine with water to form a methane hydrate.
It is not easy either to gain access to the methane hydrate or to bring the methane to the surface, but the amount of it is tremendous. There is more energy in the world's methane hydrate in the oceans than in all of the (other) fossil fuel reserves combined.
Source: Erwin Seuss, Gerhard Borhmann, Jens Greinert, and Edwin
Lausch, "Flammable Ice," Scientific American (November, 1999).