In 1870, explorers traveling in Wyoming witnessed something amazing. Water shot from the ground and rocketed more than 100 feet into the air. It happened again and again—once every hour or so. The men named the site Old Faithful. Now, 150 years later, it’s the most famous landmark in Yellowstone National Park.

Old Faithful is a geyser (GYE-zuhr). It becomes active underground as molten rock called magma heats up water until it boils like liquid in a kettle. The water bubbles to the surface and bursts into the air. As more water heats up, the process begins again.