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.