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Ogasawara NP, Tokyo
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That natural arch is a wave-cut doorway: it connects Ogi-ike to the open sea. The bowl behind it began as a limestone sinkhole, so this beautiful “pond” is actually a flooded bay.
Minamijima’s limestone began as a coral reef 34–27 million years ago. Rain later dissolved it into sinkholes and sharp ridges; after the area sank, surviving ridges became the curved reeflets scattered offshore.
Using multibeam sonar in 2026, researchers mapped submerged karst across a plateau about 4 by 2.7 km southeast of Chichijima. The arch is a glimpse of this much larger underwater landscape.
Shells scattered across Ogi-hama belong to the endemic land snail *Mandarina luhuana*. Once thought extinct 1,000–2,000 years ago, it likely survived on Minamijima into the 17th century, according to newer radiocarbon modeling.
A limestone cave on Minamijima preserved a seabird archive: sampled bones date from 6,000–600 years ago, and 3 now-rare petrel species made up over 70% of 1,318 identifiable bones.
The pale sand inside Ogi-ike is also a nesting beach for green sea turtles. After a night visit, their tracks may mark the shore—a living use of this protected bay, not just a striking geological formation.
Some seabirds on Minamijima nest in rock cracks or burrows in the ground. The marked stone route helps visitors avoid trampling these hidden homes as they explore the island.
Removing feral goats in 1971 let Minamijima’s plants recover—but then visitor footsteps began damaging them. Tokyo and Ogasawara responded with locally guided ecotourism rules, put into operation in 2003.
