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Ogasawara NP, Tokyo
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Despite its name, Ogiike (“Fan Pond”) is a bay: waves opened a tunnel through the limestone, connecting its fan-shaped basin to the open sea. The basin began as a sinkhole.
The sand beside Ogiike is a green-turtle nesting beach—and it has an extraordinary entrance. Japan’s Environment Ministry describes Minamijima as the world’s only nesting ground reached through an arch-shaped rock passage.
Ogiike’s white sand holds half-fossil shells of the giant land snail Hirobeso katatsumaimai. Radiocarbon dating puts some at just 1,000–2,000 years old—a startlingly recent past for an extinct species.
Ogiike’s limestone began as a shallow-sea reef 34–27 million years ago. Rain carved sinkholes into it; later subsidence let seawater flood them. This rare submerged karst landscape was designated a national natural monument in 2008.
Nearby In-yo-ike is a different kind of pond: brackish water there grows saltier when storms bring in seawater, then fresher after rain. Its aquatic life can come and go as conditions shift, and migratory birds likely carried in its waterweed.
After feral goats were removed in 1971, Minamijima’s plants recovered—then visitor footfall stripped vegetation along the trail again. Restoration crews replanted turf, protected soil and laid stepping stones; monitoring photos show regrowth by 2021.
