Three conditions must be met for geysers to form. There must be: an underground circuit in which the water that percolates into the ground can circulate, then rise to the surface; a reservoir, where this water can accumulate; and a nearby pocket of magma (molten rock), which heats the trapped water. Water percolates into the ground and accumulates in reservoirs near a pocket of magma. As it is heated, the water slowly turns into steam. The pressure grows, propelling a powerful jet of water and steam toward the surface. The phenomenon can last from several minutes to several hours. The jet of water subsides when the reservoir contains no more water or steam.