TIL The proposed Helsinki-Tallinn Railway Tunnel, when completed, will be the largest underground tunnel in the world. It will help to reduce time to cross the current Gulf of Finland route from tw...
This is 100km. The Bering Straight separating Russia and Alaska is 83 km (52 mi).
There have been many proposals to build a bridge or tunnel to cross it. China recently (2014?) considered construction of a "China-Russia-Canada-America" railroad line that would include construction of a 200-kilometer-long (120 mi) underwater tunnel that would cross the Bering Strait. Source (from Wikipedia)
This project is still far away from becoming reality. The Finnish railway company "VR" is somewhat of a joke among the Finns because their trains stop working if there's for example snow or leaves on the tracks