Yellowstone National Park sits atop a subterranean chamber of molten rock and gasses so vast that the region, known for its geysers and grizzlies, is arguably one of the largest active volcanoes in the world.

Granted, it's not your typical volcano, either in scale (it's huge), appearance (it's a vast depression, not a single mountain) or frequency of eruption (at least hundreds of thousands of years apart).

But it is active, and the evidence is everywhere.

A relatively close-to-the-surface magma chamber — as close as 5 miles underground in some spots — fuels thousands of spewing geysers, hissing steam vents, gurgling mud pots and steaming hot springs that help make Yellowstone such an otherworldly and popular tourist attraction, with 3 million summer visitors.

Molten rock and gas in a chamber near the Earth's surface is similarly present below "traditional" cone-shaped active volcanoes, like Mount St. Helens in Washington state.

But there are differences. Huge differences.

The crater atop Mount St. Helens is about 2 square miles. The Yellowstone "caldera" — a depression in the Earth equivalent to a crater top — is some 1,500 square miles.

The 1980 Mount St. Helens eruption blew 1,300 vertical feet off the mountain, sent an eruption column 80,000 feet high in 15 minutes, ejected 1.4 billion cubic yards of ash detectable over 22,000 square miles, and killed 57 people.

But the last major eruption at Yellowstone, some 640,000 years ago, ejected 8,000 times the ash and lava of Mount St. Helens.

And that wasn't even the largest eruption in Yellowstone's prehistoric past.

"Yellowstone is much larger than any other volcanic feature in North America," says geophysicist Bob Smith of the Yellowstone Volcano Observatory and the University of Utah. "People don't realize this."

>>> NEXT PAGE: WHAT'S FUELING YELLOWSTONE?