Son Doong Cave (Vietnam)
Credit: Joint British-Vietnamese Caving Expedition 2009 / Barcroft Media / Getty Images

Son Doong's highest and widest portions extend more than 650 feet and 500 feet respectively, making it roughly twice the size of Deer Cave, the second-largest cavern in the world. It also remained undiscovered until 1991, when a local stumbled upon it while seeking shelter from a rainstorm. Aside from massive chambers and stalagmites, Son Doong offers something few other caverns can: a jungle. A portion of the cave's ceiling has collapsed, creating an opening called a cenote, which allows light, along with plant and animal life, to creep inside. Explorers discovered Son Doong's lush, sun-soaked "Garden of Edam" almost two miles from the entrance and have since learned that this ecosystem within an ecosystem supports flying foxes, monkeys, and birds. The Vietnamese government is taking pains to keep the cave as pristine as possible, meaning access is restricted to only a couple hundred visitors this year.