Shin-Kanmon Tunnel
The 37th longest tunnel in the world
The Shin-Kanmon Tunnel is a 18.713-kilometre-long (11.628 mi) undersea railway tunnel under the Kanmon Straits connecting Shin-Shimonoseki Station on the Japanese island of Honshu and Kokura Station on Kyushu. At the time of opening in March 1975, it was the longest railway tunnel in Japan before being overtaken by in 1988 the opening of the Seikan Tunnel.
Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).