"Spelunking isn’t all about exploring dark, damp places. Case in point: Son Doong. A cave system that boasts the largest passage in the world, Son Doong was first mapped by caving experts less than a decade ago, before it was opened to the public in 2013. It’s difficult to do justice to the immensity of the system, but consider these facts: Some chambers are large enough to house a commercial passenger plane or a 40-story skyscraper; there are numerous microclimates within the complex of caves, and parts are home to subterranean jungles; and not just one but many rivers run through the network. In order to safeguard its structure and unique ecosystem, authorities limit access to this wonder—only 800 permits are granted to visitors each year."