Found on the western edge of the Tennengebirge, the spectacular World of the Ice Giants is the largest system of ice caves in the world. Covering some 30,000 square meters, the caves were carved by an underground river in the Tertiary period. Discovered in 1879, they were opened to the public in 1912, and to date, an amazing 45 kilometers of the network has been explored. After winding along the Great Ice Wall, you'll be confronted by the massive Hymir Hall with its impressive ice formations and icicles. Stone steps lead to the Eistor, or Ice Gate, a spectacular 1,775-meter-high wall of ice, and the great Ice Palace. Conducted tours last two hours, and the trip to and from the caves takes a few hours, so expect to spend the best part of a day exploring the area.