One hundred and fifty years ago, explorers traveling in Wyoming saw something amazing. Water shot from the ground and rocketed 125 feet into the air. It happened again and again—about once every hour! The men named the sight Old Faithful. It’s now a famous landmark in Yellowstone National Park.

Old Faithful is a geyser (GYE-zuhr), a landform that begins underground. There, hot rock heats up water until it boils like water in a kettle. It bubbles to the surface and bursts out. As more water heats up, the process begins again.