Southern China is home to some large caves, featuring some amazingly large chambers. A part of the Gebihe Cave System, the Miao Room is considered the world's second large cave chamber by area.
It turns out determining the volume of a cave chamber is much more difficult. Caves, unsurprisingly are dark. Really dark. According to the article, headlamps only illuminate 150 feet or so, making the true dimensions of a room extremely difficult to measure when the chambers are so large.
National Geographic describes how explorers have begun using 3D laser scans to help them more accurately map the dimensions and features of the caves.