
This is Salina Turda, a medieval salt mine that was in operation as early as 1075. It ceased mining operations in 1932.
In 1992, people began using the mine as halotherapy center, attracting tourists. The salt atmosphere is considered healthful and the air in the mine may be some of the purest on the planet due to the many layers of natural filtration.
Beginning in 2008, the Romanians undertook a massive renovation project, transforming Salina Turda into a wonderland. The park features table games, boats to paddle around an underground lake, a Ferris wheel, an amphitheater, and more, engulfed in the surreal, subterranean atmosphere of the gigantic caverns. It’s not exactly Six Flags. It’s not built for thrills, but I can’t imagine anyone coming down here without getting goosebumps.
This video shows the architecture and many of the sites. There is a wonderful gallery of photos on their web site and any image search will turn up beautiful, haunting pictures.
Here are additional photos:

Photo by Cristian Bortes from Cluj-Napoca, Romania (Salina – Turda) [CC BY 2.0], via Wikimedia Commons

By Strainu [CC BY-SA 3.0 ro], via Wikimedia Commons

Photo by Cosmin Danila [CC BY-SA 3.0 ro], via Wikimedia Commons

Photo by Cosmin Danila [CC BY-SA 3.0 ro], via Wikimedia Commons

Photo by Adymark [CC BY-SA 3.0 ro], via Wikimedia Commons