Dug deep and found this photo of the old Elbe Tunnel, one of the oldest operating tunnels in Europe. It runs beneath a river and connects the city of Hamburg to the port/shipyards at Steinwerder. Interestingly, traffic only flows in one direction the first half of the day and reverses in the second half.
You can take a walk down a winding set of stairs to reach the pedestrian paths in the tunnel or join some cars and bicycles in manually operated lifts that takes you underground (or under-river, in this case).
for more! (: