Sakhalin–Hokkaidō Tunnel

The Sakhalin–Hokkaido Tunnel is a proposed tunnel (or potentially bridge) connection to link the Russian island of Sakhalin with the Japanese island of Hokkaido through the Russia-Japan border. Cost estimates by Russia in the year 2000 put the project at $50 billion and that would span across the Soya strait for 45 kilometres (28 miles). If built as a bridge, it would have been the longest bridge in the world at the time.

Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).