Scientists have discovered a magma reservoir beneath Yellowstone National Park's super-volcano that's big enough to fill the Grand Canyon more than 11 times over.
The gigantic chamber of partly molten rock is four times bigger and much deeper than the previously known cavity just above it.
The upper chamber was responsible for three ancient volcanic eruptions that coated much of North America in ash - 2m, 1.2m and 640,000 years ago.
Researchers at the University of Utah mapped the new reservoir using seismic tomography.
They say the discovery fills a missing link in the volcanic plumbing system of Yellowstone, which straddles the Wyoming-Idaho-Montana border.
Jamie Farrell, a co-author of the study, said: "The magma chamber and reservoir are not getting any bigger than they have been.
"It's just that we can see them better now using new techniques."
Scientists said the discovery doesn't increase the risk of an eruption, which is estimated to happen every 700,000 years.
According to the US Geological Survey, such a disaster would be extremely unlikely in the next few thousand years.
The chance of an eruption is said to be one in 700,000 in any given year.
But the Yellowstone super-volcano remains one of the Earth's biggest geological timebombs.
The magma reservoir is what fuels Yellowstone's geothermal attractions such as geysers, mudpots, steam vents and hot springs.
The lower chamber is 12 to 28 miles (19 to 45km) under Yellowstone.
It has a volume of 11,500 cubic miles (48,000 cubic km), or 11.2 times the volume of Arizona's Grand Canyon.
The findings were published on Friday in the journal Science.