One hot early morning last July, archaeologist Jason De León and his team were collecting artifacts in an empty stretch of the Sonoran Desert in southern Arizona. The study area, about 55 miles south of Tucson and 40 miles north of the Mexican border, is traversed by hundreds of thousands of undocumented migrants every year. Since early 2009, De León has been cataloging the objects — water bottles, diapers, knock-off Nikes, rosaries — that the migrants leave behind on the brutal journey. But on this particular morning, his team stumbled on what he’d been dreading since day one: a dead body.

De León, 36, is one of National Geographic’s Emerging Explorers, and he described his Undocumented Migration Project