Methane hydrate is a solid crystal of ice surrounding a methane core, which exists in a low-temperature, high-pressure state at the ocean floor or in subterranean layers. Since methane can be used as natural gas, methane hydrate is sometimes called “burnable ice.” It is a clean energy, since the amount of nitrogen and sulfur oxides that occur with burning is lower than with petroleum or coal. The oceans near Japan are said to contain methane hydrate reserves of 7.4 trillion cubic meters, equivalent to about 100 years worth of natural gas use in Japan.