A lava dome discovered in the Pacific Ocean off southern Japan contains more than 7.5 cubic miles of volcanic magma and could kill as many as 100 million people if it erupts, according to Japanese scientists.

In a paper published in Scientific Reports, researchers at the Kobe Ocean Bottom Exploration Centre reported that the lava dome - one of the largest ever discovered in the world - is expanding within the Kikai Caldera, an undersea volcano just over 30 miles from the southern tip of Kyushu, the most southerly of Japan’s main islands.

The dome stands nearly 2,000 feet proud of the seabed and is now a mere 100 feet beneath the surface. It is more than six miles in diameter and occupies a massive hollow within the caldera that was created by a volcanic supereruption about 7,300 years ago.

That eruption is believed to have wiped out the prehistoric Jomon civilisation in southern Japan.

Distortions in the surface of the feature have led researchers to conclude that lava is building up beneath the dome, a suspicion heightened by plumes of superheated water and bubbling around the caldera.