Hotton, Italy
The Caves of Hotton are speleothem caves located in Wallonia near Hotton in Belgium, which were discovered in 1958 and are around 5 or 6 km long and 70 metres deep. A stream called Syphon runs at the bottom of the caves.
Listen to the audio guide for Caves of Hotton Free on WIP — starts by itself when you arrive, even with the screen off