Found via Neatorama:
In 1940, Hitler, incensed by the successful bombing of Berlin by the RAF. ordered the construction of three enormous flak towers to protect the city. Soon afterwards, this idea quickly spread around Germany.
The L Tower in Vienna is now, well, you take a guess. If your German is any good then its current name – Haus des Meeres is a complete giveaway. If not, then you may be surprised to discover that it is an aquarium. Instead of weapons of war and people huddling from falling bombs it now houses over three and a half thousand animals, with huge fish tanks containing sharks, turtles and piranhas (in different tanks one assumes). There is even a new tropical house with free flying birds and free-running monkeys.
Image courtesy of: luiscdiaz