The Magical Cavern and Kingdom of Argondian - 5 Petrinske sady, Prague, Czechia

This one isn’t really a cave and the artist is certainly not prehistoric. Prague artist Reon Argondian (formerly Jan Zahradnik) didn’t want to display his work in a boring gallery and turned an old mill building into the Kingdom of Argondia. Each rough wall is covered in colourful and psychedelic paintings and sculptures, making it into an otherworldly place. The building is in the forests on the small hill of Petrin in Prague, Czech Republic.