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.

Source: Wikipedia — Japan–Korea Undersea Tunnel (CC BY-SA 4.0)