Salina Turda was a massive salt mine in Turda, Romania. Its origins date back to the 17th century and it was a working mine up until 1932. Sixty years later the mine reopened as a tourist attraction featuring a museum, event space, walking tours and random activities like pool, table tennis and even boat rides.
Inside Salina Turda are number of mines, rooms and galleries that can be explored. The deepest is the Terezia mine which is 120 m (394 ft) deep. The underground lake is between 0.5 and 8 m (1.6 – 2.6 ft) deep. In the center of the lake there is a 5 m (16 ft) high island composed of salt waste that has been dumped there since 1880.
The interior maintains a steady 11-12 degrees celsius and 80 percent humidity, completely devoid of any allergens and an almost absence of any bacteria, making the unique micro-climate a destination for those suffering from allergic respiratory diseases. [source]