With a depth of around 2,199 metres, a cave in Georgia is so deep that nearly seven Eiffel Towers could fit inside.

The wonder was discovered in 1923, when the cave's early owners ... Within a year, the Frozen Niagara Entrance was opened, and the route was later illuminated by electric lights.

Crystal Cave, the ancient marble cavern at the end of a steep half-mile hike in Sequoia National Park, is slated to reopen to ...