Under the earth’s surface, scientists have found reservoirs that contain hot water or steam. The deeper the reservoir is, the hotter the water it contains. As water heats up, it can become hot enough to rise to the surface.

By drilling into these reservoirs, it allows the pressurized water to get to the surface. Geothermal power plants are built in these areas so they can utilize the hot water rising to the surface.

The rate at which the temperature increases with depth is the geothermal gradient. It quantifies how much heat is coming from the Earth’s interior. Power plants are built in areas with high geothermal gradients.