Geographically, geysers and hot springs serve as external indicators of subterranean volcanic heat. They are primarily found in volcanic “hotspots” or active plate boundaries like Yellowstone, Iceland and New Zealand. For these to form, groundwater must seep down near hot igneous rocks. In a hot spring, the water rises and circulates freely. In a geyser, a constricted plumbing system prevents free circulation, allowing water to become superheated. The resulting steam pressure eventually forces a violent eruption of water. These features are significant for tourism and are often exploited for sustainable geothermal energy production.