6 of 10 Creative Commons photo by AllAroundTheWest is licensed under CC BY 2.0
Lewis & Clark Caverns State Park, MT
8 of 10 Creative Commons photo by Kathryn Adams is licensed under CC BY 2.0
Mollie Kathleen Gold Mine, CO
9 of 10 Creative Commons photo by Anita Gould is licensed under CC BY 2.0
Kazumura Lava Tube, Big Island, HI
10 of 10 Stanley Chen Xi, landscape and architecture photographer/Getty Images