Sakhalin Tunnel
The 96th longest tunnel in the world
The Sakhalin Tunnel is an incomplete and currently indefinitely postponed construction project, which after completion would have connected the island of Sakhalin with mainland Russia via a tunnel of approximately 10 kilometres (6 mi) under the Nevelskoy Strait (the narrowest part of the Strait of Tartary). The construction started under the late Stalinist rule to serve primarily military and settlements purposes, and was abandoned on May 26, 1953, soon after Stalin's death.
Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).