Wieliczka Salt Mine, Poland

The Wieliczka Salt Mine, a UNESCO world heritage site in the town of Wieliczka, has produced table salt since the 13th century until mining was discontinued in 1996 due to low salt prices and mine flooding. "The mine reaches down to a depth of 327 meters and is over 300 km long. [It] features a 3.5-km tour for visitors (less than 1% of the length of the mine's passages) that includes statues of historic and mythic figures. The older works were sculpted by miners out of rock salt; more recent figures have been fashioned by contemporary artists. Even the crystals of the chandeliers are made from rock salt dissolved and reconstituted to achieve a clear, glass-like appearance. The rock salt is naturally grey, in various shades like granite, so the carvings resemble carved unpolished granite rather than having the white or crystalline appearance that many visitors expect. Also featured is a large chamber with walls carved to resemble wooden chapels built by miners in earlier centuries; an underground lake; and exhibits on the history of salt mining. The mine is often referred to as "the Underground Salt Cathedral of Poland." (source)