SCIENTISTS fear an underwater supervolcano off the coast of Japan could blow and kill 100 million people.

Experts discovered a giant lava dome in the magma champer of ancient volcano, Kikai.

5 Researchers have found a six-mile wide lava dome which they believe is sitting over a restless reservoir of lava. It lies within within Kikai Caldera, near the southerly Ōsumi Islands Archipelago Credit: Getty - Contributor

They believe it contains 32 cubic km (7.68 cubic miles) of magma, and distortions on its surface suggest it's ready to blow.

The dome is a terrifying 6.2 miles wide and almost 2,000 feet tall.

Scientists are worried the volcano could erupt for the first time in 7,300 years - spitting debris into the atmospher which could block the sun and spark a volcanic winter.

Kobe Ocean-Bottom Exploration Center (KOBEC) researchers at Kobe University said the giant lava dome was created after a caldera-forming supereruption 7,300 years ago.

The sudden explosion is believed to have wiped out civilisations in prehistoric Japan.

5 A research map of Kikai Caldera. The yellow shows magma movement along the caldera's rim Credit: Kikai University

5 Images taken by researchers on one of their six explorations to the site of the restless underwater lava dome Credit: KOBE University

Now experts fear a fresh eruption could even trigger deadly tsunamis, large enough to strike islands in Southern Japan, China and even North and South America.

The research documents say super-eruptions are 'rare but extremely hazardous events, and also have severe global impacts such as 'volcanic winter'.

It adds: "Many of these super-volcanoes repeat super-eruptions in their multi-million year histories."

It also says the dome is hissing toxic gas.

The dome is what's known as a caldera - a bowl-shaped depression which forms when a volcano collapses into itself.

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Underneath the dome is a reservoir of magma, which can reach temperatures of 1300C.

The lava dome rises to 600 metres above the seabed and is now only 30.5 metres beneath the Earth's surface.

Researchers discovered several intrusions on the surface of the dome, leading them to believe that lava is building up underneath the dome.

They also spotted super-heated water columns, near the dome's crust.

5 Scientists say a sudden eruption could kill 100 million people and spark a volcanic winter Credit: Getty - Contributor

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Professor Yoshiyuki Tastsumi, head of KOBEC and magma expert, told The Mainichi newspaper: "Although the probability of a gigantic caldera eruption hitting the Japanese archipelago is 1 percent in the next 100 years, it is estimated that the death toll could rise to approximately 100 million in the worst case scenario."

KOBEC has carried out six underwater geological surveys using submarine robots which analyse rocks, seismographs and electromagnetometers.

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Researchers are to return in March this year.

The super-volcano lies within a Japanese volcanic region known as The Ring of Fire.

Earlier this year a Japanese solder was killed when Mt. Kusatsu-Shirane erupted near a ski resort, injuring scores more people.