Helium, trapped underground for 2 billion years, is bubbling to the surface from steam vents and hot springs of Yellowstone National Park, U.S. researchers say.

Researchers with the U.S. Geological Survey say the park, located mostly in Wyoming, was releasing hundreds, even possible thousands, of times more of the ancient helium than previously thought, the Los Angeles Times reported Thursday.

About 60 tons are being release each year, enough helium to fill one Goodyear blimp every week, researchers said in a report published in the journal Nature.

Volcanic activity beginning about 2 million years ago initiated the release, they said.

That counts as a "sudden" release compared with how long the helium has been trapped within the Earth's surface, study coauthor Bill Evans, a research chemist at the USGS office in Menlo Park, Calif., said.

"[Two million years] might seem like a really, really long time to people, but in the geologic time scale, the volcanism is a recent phenomenon," he said.

Helium is produced in Earth's crust through the decay of uranium and thorium decay. This non-radioactive helium can remain trapped and build up over time, especially in places like Yellowstone where rocks have remained mostly stable for an estimated 2.5 billion years.

"The Yellowstone crust is among the oldest on Earth, and for most of its history had been part of the tectonically moribund core of North America," lead study author Jacob Lowenstern, a research geologist at the Yellowstone Volcano Observatory, said.

Then came the period of volcanic activity.

"This really isn't a volcano story," Lowenstern said. "But it reveals how the Earth's crust behaves on a long time frame. The crust 'holds its breath' for long periods of time, and then releases it during tectonically and volcanically active bursts."