Japan–Korea Undersea Tunnel

The Japan–Korea Undersea Tunnel, or 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 has been under discussion intermittently since 1917, was followed with more concrete planning during the early 1940s. It was not pursued, however, until after World War II.

In early 2008, the proposal came under renewed discussions by 10 senior Japanese lawmakers, who established a new committee to pursue it. That was followed by a study group from both countries in early 2009 that 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 late 2008, the Japan-Korea Cooperation Committee reported that the undersea tunnel could aid Northeast Asia's integration and help establish a large Asian economic sphere.

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 2010, 26 Korean and Japanese scholars proposed building an undersea tunnel to improve bilateral relations in a joint study and made specific policy proposals.

In August 2014, South Korean and Japanese business groups supported an undersea rail tunnel to boost tourism, improve relations, and generate ₩54 trillion (US$53 billion) in economic benefits, creating 45,000 jobs.

In 2021 the Korean opposition party leader Kim Chong-in expressed his support for the tunnel.

In 2022, the Japanese mood toward tunnel construction was influenced by the assassination of Shinzo Abe followed by the investigation of the Unification Church, which collected donations for the tunnel. It was further influenced following Fumio Kishida's cabinet reshuffle as it was found that over 100 lawmakers had some ties to the Unification Church. Abe and the Unification Church were 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.