Izu Peninsula

A volcanic peninsula 100–150 km southwest of Tokyo, the Izu Peninsula holds more than 30 distinct dive areas along a coastline that lies largely within Fuji-Hakone-Izu National Park. The Kuroshio Current runs close offshore, swinging water temperatures from roughly 14–16°C in late winter to about 26°C in late summer and delivering waves of subtropical migrants into temperate rocky-reef habitat each year. The west coast faces Suruga Bay — Japan's deepest at roughly 2,500 m — where the muck-and-macro mecca of Osezaki produces frogfish, Rhinopias, and winter deep-sea rarities beneath views of Mt Fuji; the east coast's Jogasaki lava terraces, formed by the Mt Omuro eruption around 4,000 years ago, hold the classic shore-entry reef of Izu Oceanic Park and Futo's ramp-and-handrail beach sites; Atami offers the 81 m Chinsen wreck; and Mikomoto Island off the southern tip serves schooling scalloped hammerheads in fast blue-water drifts from June to October. Shore-diving infrastructure — concrete ramps, guide ropes, gear carts, post-dive hot springs — is exceptional, the product of a deep domestic dive culture run in cooperation with local fishermen's unions since the early 1980s. Diving runs year-round; autumn combines lingering warm water with improving visibility, June brings the baiu rains, and typhoon swell can close exposed shore entries between August and October.

• Schools of hammerhead sharks — sometimes upwards of 100 animals — gather around Mikomoto Island off the peninsula's southern tip, with green sea turtles also resident; the encounters are fast blue-water drift dives that have made this small uninhabited islet one of Japan's most famous advanced sites.
• Osezaki faces Suruga Bay, Japan's deepest at about 2,500 m, and ranks among the country's best macro areas: guides have logged more than 30 nudibranch species on a single winter dive, and the cape acts as a seasonal cleaning station for ocean sunfish.
• The Atami 'Chinsen' is the Asahi 16, an 81 m by 18 m gravel carrier that broke in two and sank on 30 January 1986; it now lies split on sand at about 30 m (deck around 20 m), ten minutes by boat from Atami port, and is billed as Japan's largest-class recreational wreck, its hull sprouting soft corals and dense fish schools.