A massive cave recently uncovered in a remote Vietnamese jungle is the largest single cave passage yet found, a new survey shows. At 262-by-262 feet (80-by-80 meters) in most places, the Son Doong cave beats out the previous world-record holder, Deer Cave in the Malaysian section of the island of Borneo. Deer Cave is no less than 300-by-300 feet (91-by-91 meters), but it's only about a mile (1.6 kilometers) long. By contrast, explorers walked 2.8 miles (4.5 kilometers) into Son Doong, in Phong Nha-Ke Bang National Park, before being blocked by seasonal floodwaters—and they think that the passage is even...