Iwakura Kosen

Iwakura Kosen is a single-inn cold mineral spring set beside a mountain stream in Kegano-cho, Hitachiota, in the Oku-Kuji region of Ibaraki Prefecture. Tucked deep in the hills of the former Suifu Village, it is free of artificial sound and light, and on early-summer nights fireflies drift through its garden. The inn dates back roughly 120 years, and its source is said to have been discovered about a century ago.

The water is an alkaline metasilicic acid cold mineral spring rich in metasilicic acid, with a pH of 8.6 and a source temperature of only about 10C, so it is heated for bathing. Metasilicic acid is prized as a beautifying component that promotes skin metabolism, giving the bath a smooth, silky feel. The spring has long been valued for hemorrhoids, neuralgia, rheumatism, and gynecological complaints.

Today the inn welcomes overnight guests only and no longer offers day-trip bathing. Lodging with two meals runs from around 9,450 yen (please confirm), and reservations are taken directly rather than through online travel agencies. The bath looks out over the surrounding mountains, and with luck one can watch fireflies while soaking.