In the 2005 film "Supervolcano," the Yellowstone caldera erupts, disastrously spreading deep volcanic dust throughout the West. The film's tag line was, "This is a true story. It just hasn't happened yet."

The Yellowstone region really is a gigantic caldera, left by previous supervolcano eruptions. Stretching back to 4 million years ago, Yellowstone supervolcano blasts covered what's now the western half of the continental United States with inches to feet of volcanic ash, according to the University of Utah.

But could it happen again? Almost certainly not in the lifetime of anyone alive today. But a new study shows that magma deep beneath the ground surface, which could be the source of a future eruption, is rising at a record rate.

The caldera floor, the ground of Yellowstone, has risen about 3 inches a year for the past three years. The rise is more than three times greater than the fastest observed since monitoring began in 1923, the study says.

"It's a surprising finding to find that something has changed by a factor of three over historic trends," said Robert B. Smith, professor of geophysics at the University of Utah and lead author of the study. "It's very key, scientifically."

According to a study that Smith and colleagues published in the Nov. 9 edition of the magazine Science, a wide layer of magma has been rising. The hot rock drifts upward because it is less dense than the medium around it and thus buoyant; Smith compared it to a bubble going up through a bottle of honey.

Besides Smith, researchers on the team were postdoctoral associate Wu-Lung Chang, doctoral students Jamie M. Farrell and Christine Puskas of the U., and geophysicist Charles Wicks of the U.S. Geological Survey in Menlo Park, Calif.

About six miles beneath the surface, hot melted rock is moving into the magma chamber, causing the ground to deform. Most of the gigantic Yellowstone caldera is affected. The caldera is around 37 miles long by 25 miles wide; the area of ground deformation is about 37 miles by 19 miles.

The molten rock is shaped like a mattress, about 38 miles long and 12 miles wide "but only tens or hundreds of yards thick," says a U. press release.

In a telephone interview, Smith said the magma is moving in around the middle part of Earth's crust. It is by no means near the surface, he added, "but it's down where we seismically imaged a magma chamber."

Surface deformation was measured through GPS readings and measurements by a European Space Agency satellite that aimed radar at the caldera floor.