A supervolcano, that lies beneath the Yellowstone National Park in the United States, is far larger than was previously thought, say scientists.
A study shows that the magma chamber is about 2.5 times bigger than earlier estimates suggested.
A team found the cavern stretches for more than 90km (55 miles) and is 30km wide (20 miles).
In some parts it's up to 15km (9 miles) deep and it's filled with molten rock.
But experts say that it's not expected to erupt any time soon.
Prof Bob Smith, from the University of Utah, said: “We’ve been working there for a long time, and we’ve always thought it would be bigger... but this finding is astounding."
How do you measure a hidden volcano?
The team used a network of seismometers - machines that measure movement in the ground called seismic waves - that were placed all around the park.
They used the information to map areas that contained magma.
Dr James Farrell, from the University of Utah, explained: “We record earthquakes in and around Yellowstone, and we measure the seismic waves as they travel through the ground.
“The waves travel slower through hot and partially molten material… with this, we can measure what’s beneath.
The new maps how the areas holding molten rock spread much further north than previous maps show.
“To our knowledge there has been nothing mapped of that size before,” added Dr Farrell.
He explained, too, that he's certain there'll be no eruption in the near future.
When will it erupt?
The researchers are using this new information to find out whether the Yellowstone volcano is dangerous and when it will erupt again.
“Yes, it is a much larger system… but I don’t think it makes the Yellowstone hazard greater,” explained Prof Bob Smith.
He added that researchers were unsure when the supervolcano would blow again.
Some believe a massive eruption is overdue, estimating that Yellowstone’s volcano goes off every 700,000 years or so.
But Prof Smith said more data was needed, because there had only been three major eruptions so far. These happened 2.1 million years ago, 1.3 million years ago and 640,000 years ago.
The last major eruption sent ash across the whole of North America, affecting the planet’s climate.
“You can only use the time between eruptions (to work out the frequency), so in a sense you only have two numbers to get to that 700,000 year figure,” he explained.
The findings are being presented at the American Geophysical Union Fall Meeting in San Francisco in America.