The Erazm Baracz Chamber in the Wieliczka Salt Mine, Poland
The Wieliczka Salt Mine near Krakow, Poland, is the world's oldest salt mine - in fact, it's the only mining site in the world that has operated continuously since the Middle Ages!
Today, the mine still produces table salt, as it has been since the 13th century (actually the mine was in operation long before that - the earliest written account of it was dated in the 1105 AD).
The mine is deep and long: it reaches a depth of 327 meters (1,072 feet) and has over 300 km (186 mi.) of tunnels and shafts. Many of its chambers are decorated and fashioned into chapels, complete with amazing artwork sculpted from the very rock salt that made the mine.
Links: Official site (with virtual tour) | Wikipedia entry - Thanks norberto!
Previously on Neatorama: Salt Hotel in Bolivia (this post was suggested in the comment by norberto)