Just about halfway between the Old Town and the Centennial Hall, you'll find a 1893 neo-Gothic water tank turned high-tech multimedia museum dedicated - most appropriately - to the very stuff it used to hold. Divided into seven thematic parts and started off with a short 360-degree film which takes you from the Big Bang to nucleosynthesis to the formation of planets to the origin of Earth's aqua, Hydropolis is a friendly-for-all-ages discovery zone where visitors can peek at creatures found in a drop of water, spin an Archimedes' screw, sit in a replica of the Trieste bathyscaphe, which made a descent to the bottom of the Mariana Trench in 1960, or kick back in the sea-sounds-and-bioluminescent-jellyfish relaxation space. Informative and impressively designed, this is one experience we wholeheartedly recommend.

To get here, take tram number 3 from Rynek or Galeria Dominikańska to pl. Zgody, then backtrack a bit and turn right on ul. Szybka; you'll cross a small bridge before hitting ul. Na Grobli, at which point the museum will be on your right.

Average visiting time: 2hrs.