Yellowstone National Park is famous for its geysers, like Old Faithful. There are also cauldrons of boiling mud that smell like sulfur and steaming hot springs with sapphire blue pools. NPR's Nell Greenfieldboyce reports that scientists that study all of this gurgling recently discovered something unexpected bubbling up out of the ground.

NELL GREENFIELDBOYCE, BYLINE: Underneath Yellowstone National Park is a huge sleeping volcano. Bill Evans works for the U.S. Geological Survey and he says Wyoming is a strange place to find a volcano. Usually volcanoes form at the edges of the tectonic plates that make up the Earth's crust, but the Yellowstone volcano is in the middle of a plate.

BILL EVANS: Something that has not been busted up over a geologic time. It's part of the crust that formed a very long time ago, billions of years ago, and it's basically been stable since that time.

GREENFIELDBOYCE: But the plate is now moving over a hotspot, and a plume of molten rock from deep within the Earth is pushing up into these old rocks.

EVANS: They've had this boring, peaceful existence and now suddenly they're put on the front burner. They're really getting cooked.

GREENFIELDBOYCE: This cooking is doing something surprising. Evans and his colleagues looked for clues about what's happening underground by collecting the gases that rise up. Jake Lowenstern of the U. S. Geological Survey says they stick probes in the ground or hold funnels over bubbling pools.

JAKE LOWENSTERN: Just a simple plastic funnel that allows the gas to get into our tube without any air getting in, so that's important. We don't want to analyze air. Everybody knows the composition of air. They don't know the composition of the gasses that are bubbling through the pools.

GREENFIELDBOYCE: They analyze these gases back in the lab.

LOWENSTERN: And we had sort of an a-ha moment where we realized, wow, there's a lot of crustal helium coming out of Yellowstone, far more than we would have predicted.

GREENFIELDBOYCE: That's a type of helium produced by the radioactive decay of elements in the Earth's crust. And as the researchers report in the journal Nature, the amount being released by the rocks below Yellowstone is, quote, "prodigious." - each week, about enough to fill a Goodyear blimp. Evans says this must be coming from a vast store of helium that accumulated until the rock started to feel the heat.

EVANS: The gases that have been trapped in those rocks happily sitting there for hundreds of millions of years is now driven out. You know, it's kind of an interesting thought to us, how these rocks behave because it's very rare on the face of the Earth to have volcanism come into rocks that have been that stable for that long.

GREENFIELDBOYCE: The only real practical applications of this discovery are for geologists who use helium to do things like figure out the age of groundwater supplies. So don't expect to someday be blowing up party balloons with helium from Yellowstone. Nell Greenfieldboyce, NPR News.

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