Gas hydrate is a widespread combination of water and natural gases (often methane). It can be a source of problems for oil and gas drilling and production. It has potential as an energy reserve and as a basis for carbon sequestration. It is known to occur on every continent, and in large quantities in marine sediments. Its appearance and properties are like ice. There can be a layer several hundred meters thick just below the sea floor near permafrost in the Arctic. The worldwide amounts of carbon bound in gas hydrates is conservatively estimated to total twice the amount of carbon to be found in all known fossil fuels on Earth.