This ancient iron ore mine is a forest of sculpted limestone dating back 600 000 years, covering 14km of underground galleries. The abundance and diversity of rock formations make this a jewel of global geology. Two suggested routes: the first (very impressive) is a breathtaking spectacle of stalactites and stalagmites over 1.5km of galleries; the second (more adventurous) allows visitors to see some less accessible areas guided by experienced potholers (advance booking recommended).