On Mexico's Yucatán Peninsula, underwater archaeologist Guillermo de Anda made the discovery of a lifetime: a 1,500-year-old Mayan sacrificial tomb. "Going down, it's almost like a moment of meditation," de Anda says of his work exploring the region's sinkholes, also known as cenotes. "A lot of things clear out when you're hanging there looking around." In this case, a beam of sunlight revealed a skull on a ledge about 20 feet down. "The skull didn't just fall there," he says. "It was a ritual deposit." When de Anda, an expert cave diver, explored the pool at the bottom of the 70-foot-deep cavern, he found a collection of jade beads, stone tools, and a half-dozen human skeletons – suggesting the Mayans held religious ceremonies belowground as well as above. "It was amazing," he says. "You could really feel the mythology of the place."