The prefectural line, painted across the floor. Photo: Cheng-en Cheng, CC BY-SA 2.0, via Wikimedia Commons.

You can walk from one of Japan’s main islands to another in about fifteen minutes, under the sea, for nothing. Halfway along, a painted line marks where Yamaguchi Prefecture becomes Fukuoka.

The Kanmon Tunnel opened on 9 March 1958 beneath the narrow strait between Shimonoseki, on Honshu, and Moji, in Kitakyushu, on Kyushu. It has two levels: cars on top, and below them a 780-metre walkway, still open to people on foot and to bicycles and mopeds. Elevators at each end carry you down, about 55 metres on the Shimonoseki side and about 60 on the Moji side.

The floor is where the fun is. The prefectural border runs through the middle of the tunnel, and visitors line up their feet on either side of it for the photo. Walking is free; pushing a bicycle or moped through costs 20 yen. The bike has to be pushed, not ridden. Rain and wind stay on the surface.

There is a souvenir to earn, too. Stamp a sheet of paper at the stands in the elevator halls on both sides, then take it to a tourist information office, answer a short questionnaire, and you get a “Kanmon TOPPA!” commemorative certificate, in one of eight designs. Pickup points are listed on the Yamaguchi tourism site; the Kanmon Strait Tourism Promotion Council runs the scheme.

Tourism pages give the tunnel’s hours as 6:00 to 22:00, every day of the year. Hours and fees do change, so check before a special trip. Atlas Obscura’s own page refused to load for us, so the details above come from the Japanese transport ministry, Yamaguchi tourism and Crossroad Fukuoka pages instead.