Explore the ancient caves of Hang Son Doong, Vietnam. Hang Son Doong is one of the largest caves in the world and features a fast-flowing underground river. For spelunkers, exploring Hang Son Doong is the equivalent of scaling Mount Everest.
Antelope Canyon may be the most striking slot canyon. A slot canyon is a narrow canyon sliced through a mesa by the forces of nature. Some canyons measure < a yard across at the top, but drop 100 feet or more from the rim to the bottom.
The Neversink Pit Sinkhole in Alabama. The Neversink Pit Sinkhole is located near Scotsboro, Alabama. It is a natural limestone cavity leading to a cave network. The hole is 40 feet wide at its opening.