Octavio del Río, one of the cenote survey project co-directors, sketches details of a Maya skull that lies in the debris of a cenote in the northern Yucatán Peninsula. The victim may have been a human sacrifice, perhaps to Chac, the Maya god of rain, who lived in the underworld. Probing 20 or so cenotes in ancient Maya territory, expedition scientists concluded that small-town Maya followed the religious customs of grand cities such as Chichén Itzá, and that cenotes were vital to their sense of eternity.