A geyser is a natural hot spring that periodically shoots water and steam high into the air.

Geysers form in volcanic regions where underground water gets heated by hot rocks deep in the earth. The water becomes so hot that it turns to steam, building up pressure like a pot boiling over. Eventually, the pressure grows strong enough to blast the water upward in a dramatic eruption, sometimes reaching hundreds of feet high. After the eruption, the geyser goes quiet again while water refills the underground chambers and the heating process starts over.

Old Faithful in Yellowstone National Park is probably the world's most famous geyser. It erupts roughly every 90 minutes, sending about 8,000 gallons of hot water over 100 feet into the sky. People travel from around the world to watch it perform this spectacular natural show.

Geysers are rare: only about 1,000 exist worldwide, with half of them in Yellowstone. They need a specific combination of heat, water, and underground rock formations to work properly.

People sometimes call artificial water heaters “geysers,” especially in countries like South Africa and India, but those are just regular appliances that heat water for showers and washing dishes.