nature's sculpture garden filled with prehistoric boulders and stalagmites, gigantic columns by the up from calcium deposited by dripping water. >> i'm way down here about 400 feet below the top of one of the world's largest caverns. it is inside a mountain in central vietnam and was only found a couple of decades ago. after a grueling hour, an illuminating light, 1,000 feet overhead. >> every turn, there's something else. this colossal sky light where the chamber ceiling collapsed is called a doe line breathing life into this underground world. it's a lush green oasis. our camp's white sand beaches lit up below. >> it looks like avatar in here. it doesn't feel real. >> this is that hidden world we've been chasing. mountain river cave. the planet's largest.