Salt Mine in Wieliczka (external view)
The history of the Wieliczka Salt Mine dates back to the Middle Ages and is the only mine in the world which is still active to this day. The mine has 9 levels, and the deepest one is located 327 metres underground. The total length of its passageways is almost 300km. Today, it functions both as a museum and a health resort. In 1978, the Mine was entered on the UNESCO world heritage list.