Japan–Korea Undersea Tunnel

The Japan–Korea Undersea Tunnel, also known as the Korea–Japan Undersea Tunnel, is a proposed tunnel project to connect Japan with South Korea via an undersea tunnel crossing the Korea Strait that would use the strait islands of Iki and Tsushima, a straight-line distance of approximately 128 kilometers (80 mi) at its shortest.

The proposal, which had been under discussion intermittently since 1917, was followed by more concrete planning during the early 1940s. It was not pursued, however, until after World War II.

In early 2008, the proposal was a subject of renewed discussions by 10 senior Japanese lawmakers, who established a new committee to pursue it. This was followed by a study group from both countries in early 2009, which agreed to form a committee for the creation of specific construction plans. Its head, Huh Moon-do, a former director of South Korea's National Unification Board and a key member of the former Chun Doo-hwan government, said that the tunnel would help regional economics and would "play a key role in pursuing bilateral free trade talks," which are currently stalled.

In 2009, joint studies estimated high construction costs for a Japan-South Korea tunnel and concluded it would not be economically feasible supported by another South Korean study, which in 2011 found proposed tunnels to Japan and China financially unfeasible. The Korea Transport Institute estimated combined construction costs at ₩100 trillion (US$90 billion), with a very low benefit-to-cost ratio.

In 2022, the assassination of Shinzo Abe and subsequent investigation of the Unification Church, which collected donations for the tunnel, led politicians to distance themselves from the project. The Unification Church are strong proponents of the tunnel.

The proposed tunnel would be more than 200 km (120 mi) long and serve a portion of freight traffic, as well as some of the approximately 20,000 people who travel daily between the countries.