Gas hydrates are ice-like crystalline solids that form from mixtures of water and light natural gases such as methane, carbon dioxide, ethane, propane and butane. Methane was the dominant component among other hydrocarbon gases in the sediments. Gas hydrates, potentially one of the most important energy resources for the future. Methane gas hydrates are increasingly considered a potential energy resource. Enormous reserves of hydrates can be found under continental shelves and on land under permafrost. Gas hydrate or clathrate consists of three general structure types. Depending on the size of the guest molecule, natural gas hydrates can consist of any combination of three crystal structures: (1) Structure 1 or sl, (2) Structure II or all and (3) Structure H or sH. When pure liquid water freezes it crystallizes with hexagonal symmetry, but when it "freezes" as a hydrocarbon hydrate it does so with cubic symmetry for sl and all, reverting to hexagonal symmetry for sH. Methane hydrates are widespread in sea sediments hundreds of meters below the sea floor along the outer continental margins and are also found in Arctic permafrost. Some deposits are close to the ocean floor and at water depths as shallow as 150 m, although at low latitudes they are generally only found below 500 m. The deposits can be 300-600 m thick and cover large horizontal areas. Hydrates may affect climate because when warmed or depressurized, they decompose and dissociate into water and methane gas, one of the greenhouse gases that warms the planet. Methane is a greenhouse gas. Discharge of large amounts of methane into the atmosphere would cause global warming. Methane hydrates hold the danger of natural hazards associated with sea floor stability, release of methane to ocean and atmosphere and gas hydrates disturbed during drilling pose a safety problem. (C) 2010 Elsevier Ltd. All rights reserved.