Tokusa Onsen is a historic hidden hot spring located along the Nishine River valley in Minamiaizu Town, Minamiaizu District, Fukushima Prefecture, at an elevation of approximately 795 meters. Discovered around 1065 AD during the Heian period, it boasts over 1,000 years of history and has long been known as Aizu's Hidden Hot Spring. The onsen name 'Tokusa' derives from the horsetail plants (Equisetum hyemale) that once grew abundantly in the area. The communal Iwaburo (Rock Bath) is a semi-outdoor mixed bath carved from the riverbed rock along the Nishine River. Its most remarkable feature is the ashimoto yushutsu (foot-sourcing spring), where 100% pure hot spring water bubbles up directly from between the rocks at the bottom of the bath. This is an extremely rare type of hot spring, with only about 30 such facilities among over 20,000 hot spring facilities across Japan. The water is classified as a simple hot spring (low tension, weakly alkaline, high temperature) with a source temperature of 44.8 degrees Celsius. The unique experience of soaking in fresh spring water that has never touched the air is its greatest appeal. There are two bathing pools: the source springs from the back pool and flows into the front pool. Although the bathhouse has been destroyed multiple times by typhoons and flooding, it has been rebuilt each time with donations from hot spring enthusiasts across Japan. The bath is open 24 hours and costs 300 yen. A women-only changing room is available, and bathing garment rental (300 yen) is offered for female visitors.