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Yellowstone is a special place. It was the world's first national park.

Many people visit Yellowstone. They look at waterfalls. They watch hot water shoot up. They spot wild animals.

That is not all. Yellowstone also has a huge volcano. It is hidden under the park.

Yellowstone is hot. Steam rises from creeks. Sizzling springs boil. Hot mud pools bubble.

Then there are the geysers. A geyser is a natural water fountain. Yellowstone has more than 300 of them. No place else has more geysers.

The most famous geyser is Old Faithful. It shoots up water every 90 minutes or so. It is amazing.

First you hear a hissing noise. It gets louder and louder. Then water shoots up from the ground. It rises 145 feet high! Then it all stops.

All those hot spots are signs of a volcano. They are not the only ones, however. There are others.

Take the cliffs. Many are a special kind of rock. It formed when melted rock cooled down. What can melt rock? A volcano can.

Yellowstone also has many lakes. The biggest fills part of a caldera. That's a bowl-shaped hole. A caldera forms when land caves in. What can make that happen? A volcano can.

Yet no volcanoes rise above the park. So scientists looked below it.

Yellowstone's volcano lies far below the ground. It is a deep pool of magma. That is melted rock.

The magma heats everything around it. That makes strange things happen. For example, the magma causes geysers.

How does that work? Well, the magma heats the rocks above it. The rocks heat water caught in cracks. The hot water moves up and up. Then it bursts up through the ground.

The volcano at Yellowstone is large. It is one of the biggest on Earth. Scientists call it a supervolcano.

The Yellowstone volcano blew up long ago. It has not done so since. Yet it is still active.

When will it blow up next? No one knows. Scientists are watching for warning signs.

One sign is that magma will rise. That will cause bumps in the land. There are none at Yellowstone.

Earthquakes are also a bad sign. Strong quakes mean a blow-up is coming. Yellowstone has not had one.

So scientists do not expect the volcano to blow up soon. That is a good thing. A supervolcano blow-up would be nasty.

Yellowstone may not blow up for thousands of years. Until then, the volcano keeps busy. It heats rocks. It heats water. It makes an amazing place for us to go see.

Article by Beth Geiger. Top-of-page photo by Norbert Rosing/National Geographic Image Collection. "Yellowstone: A Supervolcano" appears on pages 4-11 of our March 2006 issue.

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