Not just any fish tank, this extraordinary aquarium occupies an entire 19th-century military fort – one of 55 built to defend the Austro-Hungarian Empire's main naval base. There are even sharks in an old artillery unit. The displays are well laid out and themed, with an emphasis on environmental issues. The aquarium does its bit, operating a sea-turtle rescue centre. Other attractions include rays, crabs, eels, starfish, anemones, seahorses, jellyfish, caimans, octopuses and a huge Indian python.