Natural gas hydrates occur worldwide in polar regions, normally associated with onshore and offshore permafrost, and in sediment of outer continental margins. The total amount of methane in gas hydrates likely doubles the recoverable and non-recoverable fossil fuels. Three aspects of gas hydrates are important: their fossil fuel resource potential, their role as a submarine geohazard, and their effects on global climate change. Since gas hydrates represent huge amounts of methane within 2000 m of the Earth's surface, they are considered to be an unconventional, unproven source of fossil fuel. Because gas hydrates are metastable, changes of pressure and temperature affect their stability. Destabilized gas hydrates beneath the seafloor lead to geologic hazards such as submarine slumps and slides. Destabilized gas hydrates may also affect climate through the release of methane, a "greenhouse" gas, which may enhance global warming.