The heat that drives geothermal activity in the Yellowstone area comes from brine (salty water) that is below the surface. This is actually below the solid volcanic rock and sediment that extends to a depth of 3,000 to 6,000 feet (900 to 1,800 m) and is "inside" the hot but mostly solid part of the pluton that contains Yellowstone's magma chamber. At that depth the brine is superheated to temperatures that exceed but is able to remain a liquid because it is under great pressure (like a huge pressure cooker).