Kikai Caldera

Kikai Caldera is a submerged depression of volcanic origin located beneath the waters of the Osumi Strait between Kyūshū and the Ryūkyū Islands. The rim of the structure lies mostly on the seafloor, with only Satsuma Iōjima and Takeshima islands visible above the waterline.

The depression formed through a massive eruption around 7,300 years ago that covered much of southern Japan with volcanic ash. This eruption ranks among the largest events of the Holocene and permanently altered the landscape of the surrounding regions.

The name comes from Kikai Island, located within the submerged structure and now used as a monitoring station for volcanic activity. Local fishermen know the waters above the depression for their thermal currents and occasional sulfur vapors that rise to the surface.

The submerged structure is not directly accessible to visitors, but Satsuma Iōjima island offers a vantage point over the active volcanic fields within the larger formation. Boat trips to the area depend on weather conditions and current volcanic activity, which local authorities assess.

A large dome of solidified lava rises in the center of the submerged depression and continuously releases hot gas clouds documented through underwater robot studies. This ongoing degassing shows that magma remains present deep beneath the ocean floor.