There is a rumor that Slănic salt mine will be featured in the next Batman movie as Batman's cave. By it's official name "Salina Unirea", the mine is quite impressive and spacious, having 54 meter high "rooms". It was started in 1938 and closed in 1970, the temperature is a constant 12 degrees Celsius, it's 208 meters below ground and the work was mostly done with inmates. After being shut down it has become a therapy center for respiratory diseases. If you move there you won't catch a cold ever again. The descent is done via a creepy and ancient elevator, there are also stairs but those look even creepier - I wonder how they got the Batmobil down there, if required, anyway. Probably disassembled.
The only problem is that the location is not on a very circulated corridor, you have to make a small detour from the main roads to find Slănic, but it's worth the trouble.