A bit to the north of the city, these are an underground marvel, with chambers of spectacular formations, one the size of a football field. Gigantic towering columns, emerald pools, delicate, crystalline soda straws -- all living, still growing. During the summer, the first tour of the day is an exploration of the passages by the light of cave lanterns, just as students who first discovered the place in 1960 saw it. Basic tours cost around $18 and take 70 minutes. An adventure tour with caving gear can take up to 4 hours and cost around $100.