is a massive, mostly submerged caldera

Caldera

A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...

 up to 19 kilometres (11.8 mi) in diameter in the Ōsumi Islands of Kagoshima prefecture

Kagoshima Prefecture

is a prefecture of Japan located on the island of Kyushu. The capital is the city of Kagoshima.- Geography :Kagoshima Prefecture is located at the southwest tip of Kyushu and includes a chain of islands stretching further to the southwest for a few hundred kilometers...

, Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

. It is the remains of the ancient eruption of a gigantic volcano

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

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Kikai Caldera was the source of the Akahoya eruption

Akahoya eruption

The Akahoya eruption was the strongest known volcanic eruption of the Kikai Caldera in Kyūshū, Japan. It ejected about of volcanic material, giving it a 7 on the Volcanic Explosivity Index. The Akahoya eruption is one of only six known eruptions reaching that magnitude during the Holocene, or the...

, one of the largest eruptions during the Holocene

Holocene

The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...

 (10,000 years ago to present). About 6,300 years ago, pyroclastic flows from that eruption reached the coast of southern Kyūshū

Kyushu

is the third largest island of Japan and most southwesterly of its four main islands. Its alternate ancient names include , , and . The historical regional name is referred to Kyushu and its surrounding islands....

 up to 100 km (62.1 mi) away, and ash fell as far as Hokkaidō

Hokkaido

, formerly known as Ezo, Yezo, Yeso, or Yesso, is Japan's second largest island; it is also the largest and northernmost of Japan's 47 prefectural-level subdivisions. The Tsugaru Strait separates Hokkaido from Honshu, although the two islands are connected by the underwater railway Seikan Tunnel...

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