Inubosaki Onsen Motoyu Kuroshio-no-yu (Inubosaki Hotel)

Inubosaki Onsen Motoyu Kuroshio-no-yu is the hot spring of the Inubosaki Hotel, perched on a bluff overlooking the Pacific Ocean in Inubosaki, Choshi City, Chiba Prefecture, a cape famed as one of the earliest places in Japan to see the New Year's first sunrise. The hotel successfully developed its own Kuroshio-no-yu source in 2000 by drilling 1,300 meters underground.

The water is classified as a sodium chloride strong salt spring, with a source temperature of 27C, a mildly alkaline pH of 7.8, and a flow rate of 300 liters per minute. Joint research by the Choshi Geopark and the Chiba Institute of Science used radiocarbon dating to confirm, in a peer-reviewed study, that the water is fossil seawater trapped in strata roughly 10,000 to 11,000 years old. The hotel states that some of its baths use a free-flowing (kakenagashi) source-water system, though details on dilution, heating, and circulation are not disclosed bath by bath.

Facilities include an observation-deck great bath and an outdoor bath overlooking the Pacific, along with a sauna. Day-trip bathing costs 1,200 yen for adults on weekdays and 1,500 yen on weekends and holidays.