The Miao Room in China’s Gehibe Cave complex. Analyzed with a new laser tracking system, the expanse of this single chamber was recently measured at 380 million cubic feet, making it the world’s largest underground space. To put this in perspective, the single room is nine times the size of the old Houston Astrodome. You could fit four copies of the Great Pyramid of Giza inside it, and still have space for the Washington Monument.