Shikinejima Mikawawan Kaichu Onsen

Shikinejima Mikawawan Kaichu Onsen is a wild hot spring that wells up from the seabed and cliffside rocks off Mikawa Bay (Okama Bay) on Shikinejima Island, part of Niijima Village in Tokyo. Locals are said to have long known of its existence, but no record survives of when it was discovered or how it came to be used, and no official hot-spring water analysis has ever been conducted. It gained nationwide attention after being featured on the Japanese television program 'Tokoro JAPAN,' and among onsen enthusiasts and divers it is known as a 'phantom hot spring,' reachable only when four conditions align simultaneously: a spring tide, low tide, dead-calm seas, and clear weather.

The spring is reported to be a sodium chloride brine spring, colorless and transparent with a faint smell of hydrogen sulfide and a salty taste, though these characteristics come only from visitor reports rather than an official water-quality report. The source bubbles up from cracks in the cliff rock and from sandy patches on the seafloor, with reported temperatures ranging widely from roughly 75 to 90 degrees Celsius, hot enough to scald bare skin. As the tide rises and seawater mixes in, pockets of comfortably warm water form, and visitors search these out to bathe. There is no heating, no added fresh water, no recirculation, and no chlorination; it flows entirely as untouched, free-flowing natural spring water.

Unlike a conventional bathing facility, there are no changing rooms, showers, restrooms, or parking areas of any kind. Visitors must charter a fishing boat from Shikinejima Port to approach the bay, then swim the final several dozen meters unassisted, meaning a reasonable level of swimming fitness and gear such as a life jacket or wetsuit are effectively mandatory. There is no set admission fee; instead, visitors negotiate a charter fee directly with the boat's owner. The safe window for bathing is limited to the short period around low tide, and the danger increases rapidly as the tide comes back in, so lingering is strongly discouraged.