Kikai Caldera

 is a massive, mostly submerged caldera up to in diameter in the Ōsumi Islands of Kagoshima Prefecture, Japan.
Kikai Caldera was the source of the Akahoya eruption, the second-largest eruption in recorded history and one of the largest eruptions during the Holocene (10,000 years ago to present). About 6,300 years ago or 4,300 BC, pyroclastic flows from that eruption reached the coast of southern Kyūshū up to away, and ash fell as far as Hokkaidō. The eruption produced about 150 km³ of tephra, giving it a Volcanic Explosivity Index of 7 and making it one of the most explosive in the last 10,000 years, ranking alongside Santorini, Changbaishan, Crater Lake, Kurile Lake and Tambora.
Kikai is still an active volcano. Minor eruptions occur frequently on Mount Iō, one of the post-caldera subaerial volcanic peaks on Iōjima. Iōjima is one of three volcanic islands, two of which lie on the caldera rim. On June 4, 2013, weak tremors were recorded. Shortly after, eruptions began and continued off-and-on for several hours.
According to ice cores, the Akahoya eruption may have occurred 4350 BC.