Frederiksberg Municipality, Denmark - Polarsteps
Discovered the hard way that the Cisterns are closed on Mondays, so first of all, don't make that mistake.
Anyway, the Cisterns are super cool! The two buildings above ground are architecturally stunning. Inside, the Cisterns was an underground water storage container that is no longer in use, so now it's used for art installations, weddings, school events and for tourists to wander around through the dark tunnels hitting gongs--seriously.
(Included in the Copenhagen card)
Long Weekend in Copenhagen