There are flak towers in Vienna, left over from WWII. They were built so strongly, with steel-reinforced concrete walls meters thick, that they cannot be demolished and stand now in parks. People have come to ignore them; this makes them invisible, despite their huge size, and many people have forgotten they exist.
Occasionally ideas are put forward for ways to use them. One tower now contains an aquarium and rain forest. The rain forest is in a multistoried greenhouse attached to the front. Inside it, monkeys and birds run about. They chirp and regard you with brief interest if you chirp back.
The aquarium consists of many tanks of various size, salt- and freshwater, over several stories. Sharks swim in one large tank, along with a single sea turtle. At first glance the motionless sea turtle looks dead, stuck into a corner, a jutting piece of rock holding it in place. It would be easy to imagine it miserably bored and psychotically paralyzed.
Then, from the wave-like motion of the skin along its neck, one realizes that the turtle has jammed itself into place in the corner and is letting the tank filter’s water jet give it a good massage.
After several minutes, the turtle slowly swims a lap around the tank before returning to its corner for another massage.
