1. Wieliczka Salt Mine, Poland
The Wieliczka salt mine, near the medieval city of Kraków, extends over 190 miles in length and drops to a depth of almost 500ft.
Salt was first mined here in the Middle Ages but production stopped in 2007. Its interior chambers and passageways are now open to tourists, although the site has attracted visitors since the 16th century.
There are nine levels of galleries, with caves excavated from the granite-grey salt rock and brine lakes glinting in the subterranean gloom. A series of 66 steps carved in the salt form the descent to the Chapel of St Cunegond (pictured), a church measuring some 220,000 square feet and displaying chandeliers made out of rock salt.