It is the world’s largest cave and no one even knew it existed until recently. Son Doong Cave was discovered in 1991 in the rugged mountains of central Vietnam (map). A local man by the name of Ho Khanh discovered an entrance to the cave but did not have the proper gear to go in.

It wasn’t until 2009 that the full extent of the cave became known, when a group of British cavers made the first survey of the cave. What they discovered was truly phenomenal. The cave is over 5.5 miles long with a river and some rooms big enough to fit a 40-story skyscraper. In one section of the cave, the roof has collapsed (probably more than a thousand years ago) to let in sunlight. There, the explorers found a pristine forest with all sorts of animals including monkeys and flying foxes.

If you are willing to pay around 3,000 dollars and trek through the jungle for days, a company has started trial tours. (tour link)