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Scientists fear that an underwater supervolcano off the coast of Japan that erupted 7,300 years ago is preparing to make a comeback. Experts worry that if it does, it might end up killing almost 100 million people.

They discovered a giant dome of lava in the Kikai volcano's collapsed magma chamber and believe it contains about 32 cubic km (7.68 cubic miles) of magma, and distortions on its surface, suggesting that the dome is growing. The lava dome is within the mostly submerged Kikai Caldera, south of Kyushu Island.

According to a Daily Mail report, the dome is 1,968 feet (600 meters) tall and around 6.2 miles (10 kilometres) wide.

Scientists also say that the eruption could take place anytime and without any warning. Apart from killing 100 million people, they predict that it could also trigger a 'volcanic winter'.

According to the scientists, if the lava dome erupts, it would eject huge amounts of debris into the atmosphere and could potentially block the sun and trigger a 'volcanic winter' in some areas.

Scientists predict that it could also cause a tsunami that would strike southern Japan, the coasts of Taiwan and China and then hit the coasts of North and South America.

Researchers at Kobe University with Kobe Ocean-Bottom Exploration Center (KOBEC) did the study and confirmed that the huge lava dome was created after a caldera-forming supereruption that took place 7,300 years ago. The eruption is thought to have wiped out a prehistoric civilization – Jomon, in southern Japan.

"Many of these super-volcanoes repeat super-eruptions in their multi-million-year histories," the report said. They believe that their research might help in "preparation for the next super-eruption."

Professor Yoshiyuki Tatsumi, head of KOBEC and a magma specialist, as well as the first author of the study, 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."

The research paper was published in Nature Scientific Reports.