How do Hydrothermal vents form?

Hydrothermal vents form when water underground is heated by magma until it expands or boils and bursts out of the earth's crust. "Hydro" means water and "thermal" means heat. Typically, it occurs in volcanically active places where there is abundant groundwater and magma close to the surface. One famous location is Yellowstone Park where Old Faithful (a hydrothermal vent) spouts regularly.