One of Vienna's stranger and more memorable sights: a large aquarium and tropical zoo packed into one of the colossal concrete flak towers the Nazis built across the city in 1944. Eleven floors hold sharks, rays, crocodiles, a walk-through tropical house full of free-flying birds and monkeys, and tanks of reef fish, all wrapped in the brutal grey hulk of the tower itself.
The combination of wartime architecture and marine life is the draw, and a glass-walled extension and rooftop café give a fine view over the rooftops of Mariahilf. Good for families and for anyone curious about how Vienna has repurposed an inescapable piece of its past.