Reminds me of Phantom of the Opera for some reason . . . where was this?
you have the explanation below in comments
Whoa... So cool!! It's like Phantom of the Opera or something!
O.M.G. WHAT IS THIS PLACE?! That's amazing!
you have the explanation below
this is an artificial cave that Bavarian emperor Ludvig made near his castle inside the rock. he had this swan shape boat in which he would sit and listen to the concert on the other side of the cave. he also made (well, his workers) waterfall inside the cave etc.