It's not a household name, and to be honest.....it has yet to be built....but it'll probably become another German construction fiasco (Stuttgart-21, BER, etc).
It'll be a road and rail tunnel which extends from the German island of Fehmarn to the Danish island of Lolland. It's the last point in Germany where you have bridge and land access, and you'd board a ferry to reach the Danish coastline area of Rodby (17 kilometers away) which is the last point on the Danish side with road and bridge access.
The Fehmarnbelt Tunnel would cross the 17 kilometers under the Baltic.
The idea behind the tunnel has taken decades to go from the basic idea to a real draft plan. Total cost? Nine billion Euro. It's a hefty amount and people question the necessity versus the cost.
Currently, a ferry service makes the run across. I actually rode this 50-odd minute ferry ride about a decade ago. The thing about the necessity is that there are lots of Germans who go north into Denmark for summer vacations. The ferry service is overly used in the summer period and it taxes what resources are available.
Criticism over the project? One of the major ferry services has legal issues going on, and various environmental groups are attempting to stop the project. You can view all of the issues over BER and Stuttgart-21....and they repeat here.
The odds that it'll be built? Enough permissions have been granted by the German and Danish governments....so my confidence would be that construction will start. Maybe they can beat the general track record of big German projects.
It would be nice to have a twelve-minute tunnel ride instead of the two hour wait-board-and-ferry across situation.