Kanmon Railway Tunnel

The Kanmon Railway Tunnel connecting Honshu and Kyushu beneath the Kanmon Strait was the world's first under-ocean railway tunnel when it opened, with the Kyushu-bound line completed in 1942 and the Honshu-bound line in 1944. Planners chose an underwater tunnel specifically because it would be harder to bomb. Engineers used mountain-tunneling techniques on the Honshu side and shield tunneling, borrowed from London's Thames Tunnel of 1825, on the softer Kyushu side. Test drilling had begun as far back as 1919, and Japanese engineers quietly studied comparable tunnels in the United States before construction started. Today the tunnel still carries rail traffic under more than 12 meters of water.