Sashiki Enjin-no-Yu (Yuinchi Hotel Nanjo)

Sashiki Enjin-no-Yu is a natural hot spring inside Yuinchi Hotel Nanjo, a wellness resort perched on a hill in Sashiki, Nanjo City, Okinawa, offering sweeping views of the Pacific Ocean. The name Enjin-no-Yu (bath of the ape-man) refers to the ancient fossil seawater in its source, dating back to an era some 5.4 million years ago when early hominids lived. Drawn from the resort's own well bored about 2,119 meters underground, it is a rare genuine hot spring on Okinawa's main island, where natural onsen are scarce.

The water is an iodine-bearing sodium chloride strong-salt spring (hypertonic, neutral, high-temperature), rich in iodine and calcium. Derived from fossil seawater, its salinity reaches roughly 80 percent of present-day seawater. The bath is ochre to golden in color with a salty taste and a distinctive aroma. The operator officially states it is 100 percent free-flowing spring water with no filtration, no heating, and no added water. Reported source temperatures range from about 52 to 57.5 degrees Celsius depending on the source, while the bath is kept at around 42 degrees Celsius.

The panoramic main bath overlooking the Pacific includes a cascade bath, a medicated bath, and a dry sauna (a mist sauna for women), and a private family bath is available in the annex. Day-use bathing costs 2,200 yen for adults (junior high and up, bathing tax included), 1,200 yen for elementary students, and is free for children under six. It is open 6:30 to 23:00 (last entry 22:30), with 400 free parking spaces and complimentary bath-towel rental. Guests with tattoos are not permitted in the main bathhouse.