Semonthong Waterfall, Lesotho, Africa Formed: Waterfalls form when a river erodes soft rock more quickly than hard rock and this may lead to the creation of a waterfall. The soft rock erodes more quickly, undercutting the hard rock.
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.