hydrothermal fluids
These fluids can be either water or steam trapped in fractured or porous rocks; they are found from several hundred feet to several miles below the Earth's surface. The temperatures vary from about 90° to 680°F (32° to 360°C) but roughly 2/3 range in temperature from 150° to 250°F (65.5° to 121.1°C). The latter are the easiest to access and, therefore, the only forms being used commercially.

Methane hydrate was discovered only a few decades ago, and little research has been done on it until recently. By some estimates, the energy locked up in methane hydrate deposits is more than twice the global reserves of all conventional gas, oil, and coal deposits combined. But no one has yet figured out how to pull out the gas