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Harbour with moored white boats below steep grey cliffs and a turquoise bay under low cloud — Hiradan Arch, Kozushima

Kozushima · Japan · Cave & cavern · Reef · Drift

Hiradan Arch

A huge cave-like arch dotted with yellow sponges and nudibranchs

Photo: Sgwrk · CC BY-SA 4.0

Hiradan Arch is a boat dive in the Tadanae islets off Kozushima, built around a huge cave-like rock structure. The dive reaches about 17 m (56 ft), where encrusting shells and yellow sponges line the rock. Nudibranchs are scattered over the structure, and clown coris, boxfish and gold-ribbon groupers work the rock around it.

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Where it is and how deep

Depth profile of Hiradan Arch: typical dive to 17 m (to 56 ft), deepest point 25 m (82 ft) 18 m · Open Water30 m · Advanced40 m · Recreational limit17 m25 m maxtypical dive
Depths on a shared scale to 40 m (130 ft); dashed lines mark common certification limits.

34.2076° N, 139.1914° EApproximate position — the dashed circle shows where the site lies; your operator's briefing has the mooring.

The dive

Divers drop in from the boat and descend to about 17 m (56 ft), where the arch appears as a huge cave-like structure. Its rock is crusted with seashells and patches of yellow sponge, and the dive is spent working along the structure looking for nudibranchs, including Ceratosoma trilobatum and Phyllidiella pustulosa.

The arch has a large entrance between 18 m and 10 m, and the rock above it forms a cylinder that rises to the surface. Clown coris hang out near the structure, a yellow-spotted boxfish forages, and gold-ribbon groupers and blackmouth goosefish sit among the rocks.

Marine life and season

  • Nudibranchs
  • Groupers
Month-by-month season rating for Hiradan Arch
JanFebMarAprMayJunJulAugSepOctNovDec
FairFairFairGoodGoodGoodPrimePrimePrimeGoodGoodFair

Season follows Kozushima: July to September is the top season; strong west winds leave the sea unsettled from January to March.

Sources

  1. Dive Site: Kozushima, Japan — Scuba Diver Life
  2. 神津島ダイビングポイント — Diving Shop Nangoku
  3. Tadanae-jima (祇苗島), Kozushima — OpenStreetMap

Photo credits

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