The water in the underwater cave stays at around 16°C year-round.
A facility located at one of the caves in the Inazumi Underwater Cave system in Bungo Ono City, Oita Prefecture, has become a popular spot for Japanese sauna enthusiasts.
It features a tent sauna near the cave's entrance, which means visitors get to cool off in the cave's cold water for a refreshing contrast after their sauna session. The water remains at around 16°C (60.8°F) throughout the year.
A Sauna Sanctuary
The specially designed tent sauna features a wood-burning stove, allowing for higher temperatures than typical saunas. After sweating for about 10 minutes, visitors can walk just 30 meters to the cave entrance and immerse themselves in the clear, cool groundwater. The invigorating cold plunge practically guarantees a sigh of relief.
Surrounded by the cave's rocky surfaces, small fish can be seen swimming in the pristine, naturally filtered water. After cooling down, visitors can relax on a reclining chair under the open sky.
Since its opening in March 2021, the limestone cave sauna has been a hit, with weekend reservations often filling up three months in advance. A 25-year-old sauna enthusiast from Miyazaki shares, "The sauna's high temperature and clean water make it incredibly satisfying. It's a unique experience worth sharing." He plans to post photos of it on social media.
The manager of the operating company said, "More than half of our customers come from outside Kyushu. We want them to have a great time and make Bungo Ono City a sauna sanctuary."
Discovered by a Child
This particular cave was discovered in 1976 when a local child found a "hole" while playing in a pond. Divers began exploring the cave the following year and discovered that it extended for a kilometer (0.62 miles), making it the longest underwater limestone cave ever found in Japan. The full extent of the cave system is still unknown, and research continues.
Following a 2014 investigation of the cave system, one of the divers suggested opening it to the general public for diving. After addressing safety concerns, the site opened for recreational diving and snorkeling in March 2019, becoming Japan's first location for underwater cave diving.
Oita Prefecture, where the cave is located, is renowned for its hot springs. In 2012, it even applied to trademark the phrase "Onsen Prefecture." However, promoting Bungo Ono City as a tourist destination proved challenging due to its lack of hot springs. The idea to market the city as a sauna destination emerged when local stakeholders were inspired by their experience of an outdoor sauna.
In 2020, the city established the "Onsen Prefecture Good Sauna Research Institute" and introduced saunas that use clear stream water as cold baths. By 2021, the city had declared itself "the City of Saunas." Many visitors now come to experience Japan's only limestone cave sauna. The city is also celebrated for its natural beauty, recognized as both a Japanese Geopark and a UNESCO Eco Park.
'Bathing' in Fresh Air
The sauna can be rented for two hours at ¥6,500 JPY (about $44 USD) per person, with three tent sizes available to accommodate different group sizes.
Dedicated staff members handle tasks such as pouring water over the sauna stones to create steam. The outdoor seats allow visitors to "bathe" in the fresh air and listen to the gentle babbling of the river. An indoor facility with heating and cooling units offers a place to rest.
In addition to the sauna and cold bath, the facility includes a campground, dining area, and an exhibition space featuring items from the Showa-era (1926–1989) like retro toys and movie posters. Other attractions include a large Kannon statue and an art museum, making it an enjoyable destination for families as well.
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(Read the article in Japanese)
Author: Kenji Nakano, The Sankei Shimbun
