Researchers from Brigham Young University say the supervolcanoes are no longer active, but 30 million years ago they spewed almost 2 million cubic feet of magma during a one-week period near a place called Wah Wah Springs in west-central Utah.
By comparison, this eruption was about 5,000 times larger than the 1980 Mount St. Helens eruption, they said.
"In southern Utah, deposits from this single eruption are 13,000 feet thick," study leader Eric Christiansen said. "Imagine the devastation -- it would have been catastrophic to anything living within hundreds of miles."
The eruption buried a vast region extending from central Utah to central Nevada, and traces of ash have been discovered as far away as Nebraska, they reported in the journal Geosphere.
It wasn't an isolated event, the researchers said; evidence was found of 15 super-eruptions and 20 large calderas.
The supervolcanoes have been hidden in plain sight for millions of years, hidden by the ravages of erosion and later deformation and not obvious to the naked eye because they affect enormous areas, they said.
"Supervolcanoes as we've seen are some of Earth's largest volcanic edifices, and yet they don't stand as high cones," Christiansen said. "At the heart of a supervolcano instead, is a large collapse."