A sea of chestnut trees blossoms down below.

A pretty local girl (Ka, my wife) enjoys the sunny weather atop the former flak tower.

Here we see a section of the two-meter-thick wall where it has been cut through with a saw. This is part of the passage from the rainforest area of the Haus des Meeres to the aquarium inside.

One of the dozen or so monkeys that live in this micro-rainforest in downtown Vienna.

Sea anemones in an aquarium tank where once a war was raging.

An alligator inside, like a green jack o' lantern.

The flak tower in all its glory, looking a bit like Mickey Mouse. We see a climbing wall here, with some ropes on it. The top part that juts out, including Mickey's ears, is now an observation deck.

The Haus des Meeres rainforest area on the left, built into the side of the flak tower.

The way in to the Haus des Meeres, the House of the Seas, the aquarium in the flak tower. Öffnungszeiten: Openingtimes täglich 9-18 Uhr: daily 9 AM-6PM Letzter Einlass 17.30 Uhr: Last inlet 5:30 PM. Hai & Piranhafütterung: Shark & Piranhafeeding, Wednesday+Sunday 3 PM. Reptilienfütterung: Reptilefeeding, Sunday 10 AM. Schlangenstreicheln: Snakestroking, Wednesday 2 PM.

What do you do with a flak tower that the German army built in 1944 in the center of your city? Put an aquarium and a rainforest in it, of course. This is one of four or five towers built to shoot down British and American planes over Vienna. They're very solidly built and it's easier to leave them where they are than tear them down. They seem symbolic of Austria's Nazi era: hulking, ugly presences; sinister; you can't wish them away, but have to try to find some way of accommodating them. This one went through my internal poetry machine as I was falling asleep the evening we visited it. In my mind's eye, it morphed into a sad monster hunched up in a myth, unable to move; then into an octopus spawning other octopi. The next morning before awakening completely I was in a semi-dream where the whole thing still seemed a locus of evil. A few years ago I used to transfer from one bus to another on a corner across from it. I spent a fair amount of time pondering it. On the uppermost part, in big letters, it says "Smashed to pieces (in the still of the night)" and the same thing in German: "Zerschmettert (im Frieden der Nacht)." This is a work by the American conceptual artist Lawrence Wiener. A modern-day mark of Cain, perhaps. Another of the city's flak towers has what I think is a pre-Colombian ceramic figure from Mexico painted on the side. Everything changes. Everything flows.