Sondoòng Cave located in Vietnam was originally discovered in 1991 by a local man. However, fear of the sounds from its underground river prevented any exploration. It wasn't until 2009 that British scientists embarked on an expedition to fully explore this mammoth cavern. The subterranean passage carved by a river over 1 million years ago can hold 40-story buildings for a half-mile block. 2 hundred meters down, you'll find the largest cave jungle on the planet. Don't forget to charge up before you go down. You don't want your head lamp going out before you get a chance to explore the raging river and under-ground forest in this giant hole in the earth. What do ya say? Bucket list worthy?