Nanki-Katsuura Onsen

At Kii-Katsuura Station you follow signs down toward the harbor, and within a few minutes you are standing at a small pier looking out at Katsuura Bay. The water here is very deep and very blue, and scattered across it are a handful of small rocky islands covered in dense subtropical vegetation that grows right down to the waterline. A few of these islands have buildings on them, accessible only by the short ferry boats that run throughout the day from the tourist pier. This is how Nanki-Katsuura's most distinctive hot spring hotels are reached, and the five-minute crossing changes the character of the visit before you have even entered a bath.

The water in the Nanki-Katsuura area is predominantly sodium chloride, drawn from over a hundred springs that dot the rugged coastline and its surrounding terrain. It is clear and carries a mild saltiness that coats the skin without being heavy. What sets this area apart from other coastal onsen towns is not the chemistry of the water but the architecture of the bathing. Hotel Urashima, on the Urashima Peninsula, houses the most talked-about bath in the region: the Bokido, a cave formed naturally in the cliff face at sea level, open on the ocean side, where you can sit in the hot water and watch the Pacific moving just a few meters below. The cave amplifies the sound of the waves so the whole bath hums with a low, rhythmic reverberation. On rough days, sea spray drifts in from the opening.

On Nakanoshima Island, reached by a separate ferry, Kumano-Bettei's outdoor terrace bath looks directly across the bay toward the mainland hills. In the late afternoon the water turns from green to dark blue and the light on the far cliffs goes orange, and you are in a hot bath in the middle of this, which is one of those situations that Japanese travel writing correctly refuses to overexplain. The island has a human population most easily counted on one hand outside of the hotel staff, and the quiet that comes with that is absolute after the last ferry leaves for the evening.

The town of Kii-Katsuura itself is worth more than a night. It is one of Japan's most important tuna ports, and the morning market at the harbor runs on a scale that makes ordinary fish markets look modest. Whole bluefin tuna are lined up on pallets while traders move between them making quick, precise bids. Eating tuna in a town this serious about tuna, then crossing the harbor by boat to a cave in a cliff face full of hot seawater, is the kind of two-part day that justifies the four-hour train from Osaka without any qualification.