Although it is hard to believe, according to a list on WIKI there are/were 16 different tunnels under the Spree river, also for the difficult subsoil of glacial valley of Berlin.
Among the U-Bahn tunnels and the working-service ones, there was the tram tunnel in Treptow, used to connect Stralau with Treptow, but it was badly damaged during the WWII and unusable. The ramp have been removed, the condition of the tunnel is largely uncertain.
The only pedestrian one in use is the one just at the beginning of the Müggelsee in Friedrichshagen connecting it to Müggelschlößchenweg, on the Müggelberge.
On one side of it there's Friedrichshagen, which is a nice area with a very traditional and not turistic flohmarkt on Sunday, nice houses and some relaxing atmosphere, a small park with some nice villas and statues.
One the other side the tunnel brings directly into the forest and the river bank, so clean air and relax in the wilderness (although there's a street for bikes and the main street is not so far).
The tunnel, which was built in 1926, is very deep and damp, in the weekends is popular and it's a good compromise to arrive to Friedrichshagen (with the S-Bahn S3) and then ends up into a nice forest walk.
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