Horikawa Onsen

Tucked away in the rural hills of Takahagi City, Ibaraki Prefecture, about a 10-minute drive from the Joban Expressway's Takahagi IC, Horikawa Onsen is a fully private, reservation-only lodging where a single group has exclusive use of the entire property. The current operator, Terme Japan Co., Ltd., continues to protect a family hot spring passed down as an inheritance, running the property as an unhosted minpaku that accommodates one to eleven guests at the same rate.

The grounds hold two self-owned sources: a sodium-sulfate spring that flows naturally at 32 degrees Celsius and 150 liters per minute, and a sulfur spring that flows at 20 degrees Celsius and 3,000 liters per minute. Both are served completely untreated, without dilution, heating, or chlorination, making Horikawa reportedly the only undisinfected, source-fed hot spring in all of Ibaraki Prefecture. Because the sodium-sulfate spring contains no tritium, it has been certified by Japan's Ministry of the Environment as background water for radiation research, drawing more than twenty institutions each year to collect samples. The sulfur spring itself is clear and colorless when fresh, clouding to a milky white after about two days, and carries a sulfurous scent reminiscent of Kusatsu Onsen, with a neutral pH.

Guests can choose between an eight-mat tatami room or a connected two-room suite combining eight- and six-mat rooms, and the property also features a sauna that uses the sulfur spring water for its cold plunge bath. Dining options range from catering by an affiliated French restaurant using local ingredients to barbecue setups or in-room meals. The property is available from a 15:00 check-in to a 10:00 checkout the next morning, and operates from April through the end of November, closing for winter from December to March. Rates, which vary by group size and season, are listed on the official Reserva booking site, with payment required in advance.