Hang Son Doong — which roughly translates to Mountain River Cave — is the world's largest cave and it is so large that it could hold a modern-day skyscraper inside of its caverns, and has its own small jungle.

Located near the Vietnam-Laos border, the cave was found by a local man named Ho-Khanh in 1991. The locals, it is said, were too afraid of the cave to go exploring because of the sound coming from the fast-moving underground river, as well as the huge vertical drop. 

In 2009, a group of scientists from the British Cave Research Association began an extensive survey of the cave's depths. According to Howard Limbert, the man leading the survey, the cave is five times larger than Phong Nha, which once held the title of the largest in Vietnam. The biggest chamber, his team found, is over five kilometers long and 200 meters tall.

The jungle found inside of the cave has formed underneath a collapsed roof in one of the caverns. After the roof collapsed, enough light spilled into the cavern that vegetation was able to creep in slowly from outside. As the vegetation took hold, larger and larger plants began to grow, and now hornbills, flying foxes, and monkeys dwell in its branches. 

In 2013, public visits started to be offered by Oxalis that offer the chance to spend days inside exploring, as well as camping, in this massive cave.