Son Doong Cave, which was just recently recognized as the largest cave in the world, is finally open to guided tours. No one knew it was the largest until British explorers first explored its depths in 2009. The Vietnamese cave was discovered by local man Ho Khanh in 1991, but he could not descend into the cave. The British expedition uncovered incredible natural wonders within the cave, like towering stalagmites, rare cave pearls, a river, underground jungle ecosystems, cave-dwelling species and ancient fossils. Some caverns of the cave, which is 9km long, 200km wide and 150km tall, can fit entire skyscrapers or city blocks. Tours through the cave can cost more than $3,000.