Mammoth Cave. Cave City, Kentucky What Lies Beneath: The world’s longest cave. Over the years, “mummies” of prehistoric cave explorers have been discovered. The first recorded find was in 1935: a six-ton boulder with a body pinned underneath. In the mid-1800s, a doctor who (incorrectly) believed the humid conditions might be a remedy for tuberculosis, turned the cave into a sanitarium.
The Krubera Cave ~ the deepest known cave on Earth, located in the Arabika Massif of the Gagrinsky Range of the Western Caucasus, in the Gagra district of Abkhazia (Georgia’s breakaway republic). This is the only known cave deeper than 2,000 metres -- from entrance to deepest explored point = 2,191 metres (7,188 ft)
Nankoweap Canyon, Grand Canyon National Park Photograph by Ralph Lee Hopkins The Colorado River winds through Nankoweap Canyon in Grand Canyon National Park, Arizona. North America's geologic history is writ large in the sandstone heights across the U.S. West.