Orsova, Romania: Veterani Cave

This looks rather innocent from the Danube, although a cave seems a little out of place. But the descriptions by actual visitors are quite enthusiastic. The water goes in about 500 meters, and opens nicely to a big ceiling. 

The interesting part is that this cave was a hiding place for soldiers for centuries. The Turks, Romans, Austrians and Gauls all used it at one time or another. This is named after General Veterani, who was in charge of the 500 troops who spent 45 days here in 1692.

Keep in mind, of course, that the water level was lower then, and the cave was correspondingly bigger.

There's even a kiosk for visitors. I didn't get to go inside, but you can.