A sign for a cave? Intrigued. It turns out that it leads to the "Cave Bar More", one of the bar's listed in Business Insider's 50 incredible bars you should drink at in your lifetime list.
The Cave Bar More is a bar underneath the More hotel in Dubrovnik, Croatia. The cave was found during the construction of the hotel, and sports among other things a high vertical cave shaft, with a glass floor in the middle to view the water at the bottom of the cave.
The vertical room:
And the sea outside is... wonderful: