Yucatán's porous limestone bedrock doesn't lend itself to the formation of perennial surface water such as rivers or lakes, and the rain that does fall in the region quickly percolates deep into the limestone. Over time, the mildly acidic rainwater eats away at the peninsula's fractured limestone bedrock, creating underground caves. Eventually, the roofs of these caves collapse, exposing subterranean water sources ranging from small caverns that link into vast underwater cave networks to large, sun-filled basins that can begin a hundred feet below the surface.
The Maya in the northern lowlands, which encompasses parts of the present-day states of Yucatán, Quintana Roo, and Campeche, relied on cenotes (a Spanish corruption of the Yucatec Mayan word for sinkhole, dzonot) as their primary source of water. According to tradition, caves and cenotes are also the home of Chac, the Maya god of rain, as well as the entrance to Xibalba, the Underworld. In times of drought or stress, or when, Maya leaders appealed to Chac by making offerings to him in cenotes.
By the time the Spanish arrived in the Yucatán and recorded the practice in the sixteenth century, the Maya had been performing human sacrifice for at least a thousand years. Bishop Diego de Landa, who recorded acts of sacrifice at Chichén Itzá in the sixteenth century, wrote:
Into this well it was their custom to cast living men as a sacrifice to the Gods in times of drought; and it was their belief that they did not die, although they never saw them any more. They also threw in many other things of precious stone and articles which they highly prized.
Click here for other contemporary descriptions of Maya human sacrifice in cenotes.
This is not to say, however, that all cenotes contain ritual offerings. Archaeologists believe the Maya kept their "ritual" cenotes separate from their "domestic" cenotes. Spiritual considerations aside, the practicalities of disease prevention alone would prevent communities from contaminating their drinking water with human remains. The challenge for archaeologists is to determine which cenotes were used by the Maya for domestic purposes--these usually just contain pots, construction materials, and some animal remains--and which ones were used to make appeals to Chac. "Human remains in a cenote are usually a good sign that it was used for ritual purposes," says de Anda. "Of course, there are always accidents," he adds, "but when you have a number of remains, the possibility that they're all accidents is obviously not likely."