Methane hydrates (also know as gas hydrates or clathrate hydrates) are a crystalline solid where the methane molecules are each trapped in a cage of frozen water molecules. Methane hydrates are stable in ocean floor sediments at water depths greater than 300 meters, and where they occur, are known to cement loose sediments in a surface layer several hundred meters thick. There are also considerable quantities of methane hydrates found in Arctic permafrost though the total is thought to be only a small fraction that found in the ocean. The worldwide amounts of carbon bound in methane hydrates is conservatively estimated to total twice the amount of carbon to be found in all known fossil fuels on Earth. Hydrates require both low temperatures and high pressure to form and eliminating either condition can free the gas from its icy cage. Sudden methane hydrate release has been proposed as the cause of some previous global warming events and may also weaken the seafloor, causing submarine landslides and tsunamis.
Plans for exploitation of methane hydrates are advancing swiftly. A series of drilling tests have been conducted over the last decade on the permafrost in Canada by the Geological Survey of Canada and the Japan Oil, Gas and Metals National Corporation (JOGMEC). More recently a joint project involving the US Government and Chevron drilled a well in the Gulf of Mexico to explore for hydrates. Last year the US Department of Energy (DOE), in conjunction with ConocoPhillips and the JOGMEC, ran a 30 day extraction test on a well in Northern Alaska. Now JOGMEC has begun drilling for methane hydrates off the south coast of Japan. This will be the first attempt to produce gas offshore. The greatest threat from methane hydrate lies in the fact that they are both highly unstable and exist in very large quantities. Extraction techniques will involve destabilising the hydrates and the potential for uncontrolled release of methane, potentially accompanied by massive undersea landslides and tsunamis, will always be present. Because methane is such a strong greenhouse gas if it escapes directly to the atmosphere, the consequences would be dire.