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.
The beautiful Perito Moreno Glacier in Argentina. In recent years, trekking tours on the ice is increasingly popular. The two standard tours are a "mini-trekking" option- a short walk of about an hour and a half, and a "big ice" version- a five-hour trek.