The ultimate source of heat for the water in a geyser is the heat from the Earth's interior, which is referred to as geothermal heat. This heat is generated by the decay of radioactive elements deep within the Earth's mantle and crust, as well as from residual heat left over from the planet's formation. The geothermal heat causes water to be heated and rise through cracks and fissures in the Earth's crust, leading to the formation of geysers and hot springs. As the heated water reaches the surface, it erupts periodically, creating the spectacular displays associated with geysers.