The floor of Yellowstone National Park’s gigantic volcano has been rising at a record rate in recent years, probably due to an underground blob of molten rock more than 14 times the size of Billings, according to a new study. The Yellowstone caldera rose nearly 3 inches a year for the past three years, faster than anyone has ever recorded. “These are rates three times (greater) than previous historic rates,” said University of Utah seismologist Bob Smith, a lead author of the study to be published in the journal Science today.

But that rapid rising isn’t an indication of an imminent volcanic eruption or hydrothermal explosion at Yellowstone, he said. It appears in line with behavior at other volcanic craters that rise and fall for thousands of years without large-scale, catastrophic eruptions. “These things go up and down,” Smith said. “That’s very common for calderas globally.”

The researchers believe the 463-square-mile slab formed from molten rock in the magma chamber beneath Yellowstone, causing the surface of the caldera to rise.