This bunker was built in WW2. An unused subway tunnel was 'upgraded' with several meters of concrete. As there is no drain or something, any water getting in there is bound to remain in there. The water is about 30 cm high and the ceiling is covered with droplets of condensing water everywhere. After the war the bunker was crossing under the Berlin wall and Eastern Germanys authorities built a wall in the bunker as well, when they found out that people used it to escape to West Berlin.