One Fathom Bank

A shallow offshore seamount rising to 6 m off Yakushima's southeastern coast, One Fathom Bank is the island's premier site for pelagic encounters. The seamount's exposed position catches full Kuroshio Current flow, concentrating planktivores and their predators. Schools of bigeye jack and yellowfin trevally are resident; in summer hammerhead sharks (Sphyrna lewini) and whale sharks (Rhincodon typus) pass through on Kuroshio eddies. The hard coral coverage on the summit plateau is dense — predominantly Acropora and Porites — with visibility regularly exceeding 25 m on slack current. Sea turtles rest on the plateau surface and loggerhead turtles use the seamount as a navigational waypoint during their nesting migrations to nearby Yakushima beaches. Strong currents on the bank's flanks can make this site challenging; most operators plan dives around neap tides.