Iwojima Higashi Onsen

Higashi Onsen is a wild hot spring, or noyu, on the shore of Iojima, commonly called Satsuma-Iojima, in Mishima Village, Kagoshima District, Kagoshima Prefecture. Iojima is a volcanic island of 11.74 square kilometres with a population of around 120, dominated by Mount Iodake, an active volcano of 703 metres that still vents white fumes. The island forms the northern rim of the Kikai Caldera, source of a colossal eruption some 7,300 years ago, and together with Takeshima and Kuroshima it makes up the Mishima Village Kikai Caldera Geopark. It is also identified with Kikaigashima, the island of exile to which the monk Shunkan was banished in the late Heian period, and sites connected with him are scattered across it. No founding date is recorded for Higashi Onsen: the water simply rose from the coastal reef, and the rock was hollowed out to make tubs.

Mishima Village and the Kagoshima tourism authority both describe the water as a sulfur-alum spring, and it is strongly acidic. An analysis of 31 samples taken at Higashi Onsen between 1961 and 2000 by Hayao Sakamoto of Kagoshima University (Nature of Kagoshima, vol. 41, 2015) records a pH of 1.6 to 1.8, a geometric mean of 1.7, and source temperatures of 47.0 to 55.8 degrees Celsius, geometric mean 53.1. Sulfate averages 4,920 milligrams per litre, roughly four and a half times the chloride content, while aluminium dominates the cations at around 400 to 960 milligrams per litre, with iron sometimes exceeding 100. The numbers explain the alum in its old name. The water shifts between a bluish milky white and green, and carries the smell of sulfur. Nothing is added or removed: no heating, no dilution, no recirculation, no disinfection.

The bathing area consists of three tubs cut into the wave-battered reef and stepped down toward the sea. The uppermost, nearest the source, is the hottest, and the water cools as it spills from one pool to the next, so bathers pick whichever suits the day. There are no changing rooms, showers or toilets; only a low wall of stacked stones above the pools, with no roof and no division by gender. Bathing is free and mixed, and swimsuits or wrapped towels are accepted. Nobody is stationed here, but local residents clean the pools regularly, so it stays reasonably tidy for a wild spring. In June 2023 damage left the tubs unable to hold water and the spring was closed for a time, but Mishima Village had it repaired by the end of July that year.