Ice cave in the Skaftafell National Park in Iceland. Ice caves collapse and new ones form as each glacier grinds slowly towards the sea. Local guides can help you (without dying) find one. (Visit Skaftafell National Park in Iceland.
Ruby Falls is a high underground waterfall located within Lookout Mountain, near Chattanooga, Tennessee in the United States. - Ruby Falls and the larger Lookout Mountain Caverns complex have been designated a National Historic Landmark.