Goh Kun, a former prime minister of Korea, is proposing a Japan-Korea tunnel as part of his campaign for president. With this tunnel intact, Japan and Korea would be directly linked by rail and highway, and assuming that North Korea comes out of its isolation in the future, it would be possible to ship goods between Japan and Europe entirely by rail (through the trans-Siberian).
This is hardly a novel idea. Back during World War II, the Japanese government had a long-term goal to run high-speed rail service from Japan through Korea and into the Asian mainland. Transport historian Roderick Smith:
The need for expansion of capacity [in the Tokyo-Osaka-Fukuoka corridor] was recognised, and work actually started on a new standard-gauge (4 ft 8 1/2 in. or 1,435 mm) line in 1940. A key part of the motivation behind this new line was to link Tokyo with the western part of Japan, which, in turn, linked up with Japanese-held territory in China and Korea. It was planned that fast electric trains, already nicknamed dangan ressha (Bullet Trains), would speed along this line towards Kyushu and perhaps even through an undersea tunnel to the Asian mainland via the Korean peninsula. Although the undersea Kanmon tunnel was completed between Honshu and Kyushu in 1942, thus directly linking two of Japan’s four main islands for the first time, the Pacific war had started in 1941 and it was to be some time before the railway network could be further expanded.
A few of the tunnels blasted as part of this plan were eventually used for Japan’s first high-speed railway line, the Tokaido Shinkansen, which opened in 1964.
Anyway, they could be on to something with this tunnel. Besides freight, an overnight high-speed train from Tokyo to Seoul could prove very popular, and in the future, it could even be extended to Beijing or farther. A big investment, sure, but perhaps not as hare-brained as it might initially sound.