We know, of course, that the reason for all of the intriguing geological formations and geysers around Yellowstone national park is the history of volcanism in the area. Well, is the pressure building for another big blow?

Yellowstone National Park, once the site of a giant volcano, has begun swelling up, possibly because molten rock is accumulating beneath the surface, scientists report.

But, "there is no evidence of an imminent volcanic eruption," said Robert B. Smith, a professor of geophysics at the University of Utah.

Many giant volcanic craters around the world go up and down over decades without erupting, he said.

Smith and colleagues report in Friday's issue of the journal Science that the flow of the ancient Yellowstone crater has been moving upward almost 3 inches per year for the past three years.

That is more than three times faster than ever observed since such measurements began in 1923, the researchers said.

"Our best evidence is that the crustal magma chamber is filling with molten rock," Smith said in a statement. "But we have no idea how long this process goes on before there either is an eruption or the inflow of molten rock stops and the caldera deflates again."

It has been 642,000 since the last major eruption, but scientists tell us not to worry about an imminent cataclysm. Still, such swelling was detected at Mount St. Helens before it erupted in the 1980s, and the three documented major eruptions in the last 2 million years all exceeded that event. Scientist do keep a close watch on the area, so we should have plenty of warning before a major event does occur.