Researchers mapped the area – which covers 700,000 sq miles, around the same size as Mexico – using a massive network of seismic sensors.
And they fear it could cause unprecedented environmental damage if emitted into the atmosphere.
The study was carried out by a team of geologists from top London university Royal Holloway.
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The area covers 700,000 sq miles, around the same size as Mexico
Lead author Dr Hier-Majumder said: "It would be impossible for us to drill far enough down to physically 'see' the Earth's mantle.
"So using this massive group of sensors we have to paint a picture of it using mathematical equations to interpret what is beneath us.
"Releasing only one per cent of this CO2 into the atmosphere will be the equivalent of burning 2.3 trillion barrels of oil."
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Scientists fear it could cause unprecedented environmental damage
The area covered by the study includes Yellowstone National Park, where previous research has uncovered evidence of a supervolcano.
The volcano at the national park – which last erupted 640,000 years ago – sits atop a huge reserve of molten rock.
It releases around 45,000 metric tonnes of carbon dioxide each day.
The area covered by the study includes Yellowstone National Park
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The Yellowstone volcano
Thu, January 7, 2016
The volcano at Yellowstone National Park in Wyoming and Montana sits atop a huge reserve of magma and last erupted 640,000 years ago
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The Sunset Lake hot spring at Yellowstone National Park, Wyoming
Experts have previously claimed it could spark a "nuclear winter" if it erupts.
According to a chilling report released by the How Stuff Works website in 2015, it could "kill as many as 90,000 people immediately".
The eruption could also "spread a 10-foot layer of molten ash as far as 1,000 miles from the park", the report added.