The Yellowstone supervolcano, formed because of geological plate tectonics, erupted most recently about 640,000 years ago. Meanwhile, the floor of the caldera rises and drops like, Smith said, "a living, breathing system."

The system's heat is what causes the famous geysers and hot pots of Yellowstone National Park.

Asked what the chances are for a gigantic eruption in the lifetime of people alive today, Smith said, "Exceedingly unlikely."

Before that happened, earth scientists would expect many earthquakes. Yellowstone has been the location of mighty quakes in the past, such as the 7.3-magnitude 1959 Hebgen Lake disaster in Montana that killed 28 people. But lately, the ground hasn't been as shaky.

Seismicity "has in fact relatively decreased in Yellowstone over the past two years," after the magma rise started. If an eruption were due soon, he would expect much more seismic activity, and for it to be based at a much more shallow level, two to five miles below the surface.

"We'd have deformation and it might be more concentrated" and happening at a faster rate, he said.

The supervolcano might not be about to go off, but "this is exciting science," said Smith.

"This is the first observation of something this big."

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