Methane hydrates are ice-like substances in which gas molecules are held within "cages" of water molecules. They form at low temperatures and high pressures where methane and water are available and are found beneath the sea floor in water depths greater than 500 metres. Large amounts of methane hydrate are thought to exist within sea-bed sediments, particularly beneath the continental slopes. The stability of this methane store may be sensitive to climate change, particularly in permafrost regions. This means that during global warming, hydrates constitute a geohazard especially where melting of the hydrate may significantly reduce the strength of the host sediment. Additionally, methane released during such events may itself contribute to global warming.