Deep in the Quang Binh province in Vietnam lies Son Doong Cave, the largest cave in the world. It was first discovered by a local man called Ho Khanh in 1991, and later explored by British explorers in 2009.
The explorers found incredible natural formations nestled in its cavernous walls, like rare cave pearls, an underground jungle, a river, gigantic stalagmites, cave-dwelling animals, and ancient fossils.
Son Doong Cave, which is believed to be between two to five million years old, measures a staggering 200km wide with a height of 150km and a length of 9km. It is so large that some caverns are able to fit entire skyscrapers or city blocks.
It is open for tours, but only for those with deep pockets; each tour will set you back at least $3,000.
View more stunning images of the cave below.
[via DeMilked and National Geographic, images via Carsten Peter for National Geographic]