The Spree Tunnel Stralau-Treptow connected Stralau and Alt-Treptow. The so-called Knüppelbahn, an old Berlin tram line, ran through it. The tunnel runs between the tunnel road on the Stralau peninsula and today's Alt-Treptow/Puschkinallee area in Treptower Park (square at the Spree tunnel) and crosses under the Spree. The tunnel still exists, but the ramps have been filled in and the tunnel has been flooded.
It was the first underwater tunnel in Germany.
The tunnel is 454 meters long, 3.76 meters wide and the apex of the tunnel section is twelve meters below the Spree, which is 195 meters wide at this point. The passage took about two minutes and was considered an attraction in Treptow until traffic was stopped.
At the trade exhibition in 1896, the first section of the tunnel could be visited on foot. On December 18, 1899, the Berlin Ostbahn began regular service through the tunnel as Germany's first public underground railway between Schlesischer Bahnhof (today Ostbahnhof) and Treptow passenger numbers continued to fall. During the 1936 Summer Olympics in Berlin, the Spree Tunnel was secured and reopened to pedestrians. During World War II, a temporary air raid shelter was set up in the northern section of the Spree Tunnel (Stralauer side). Since it was not possible to restore it in the post-war period, the tunnel was completely flooded in 1948 to prevent it from collapsing. In 1968, the access road and the first tunnel meters on the Treptow side were removed and the access ramp on the Stralau side was filled in. In December 1996, as part of preliminary investigations for the planned new development of the peninsula, the Stralau access was uncovered again for a few days and the water from the Berlin fire brigade pumped out in the area of the former air raid shelter.
Only the street name Tunnelstraße (Stralau) and the place at the Spree Tunnel (near Zenner in Treptower Park) remind of the now no longer visible Spree Tunnel.
Source: Wikipedia