A flooded sinkhole in southern Mexico so frightens nearby villagers that they won't go anywhere near it. The ancient Maya seem to have kept their distance too.

A recent underwater survey in the cavern, or cenote, located in Mexico's Yucatán, has found a likely reason for its fearsome reputation—the floors of its two chambers are littered with human bones.

To investigate the cenote, archaeologist Bradley Russell and his team spent two weeks last August diving into its submerged reaches. Russell received support from the NGS/WaittGrants Program for this work.

The photos in this gallery reveal what they found.

A rappel of 40 feet (11.5 meters) took Russell from the lip of the cenote (photo below) to the water's surface within the cave. On the way down he passed massive stalactites and the long roots of thirsty trees stretching toward the moisture below.

The locals have named this natural well Sac Uayum (pronounced sock-wye-OOM).

View Images Photograph by Rait Kütt

The cenote sits just outside the ruins of the ancient Maya city of Mayapán, about 25 miles (40 kilometers) south of Mérida, the capital of the Mexican state of Yucatán.

In its heyday, between 1150 and 1450, Mayapán was a major political center with at least 17,000 residents and a stone wall that enclosed 1.5 square miles (4.2 square kilometers) of the city.

Russell was especially intrigued by the location of Sac Uayum. Mayapán's wall curved in a way that put the cenote just outside the city limits, then turned back to include a nearby cenote known as X'coton (pronounced eesh-coh-TOHN).

"The rest of the wall doesn't zigzag like that," Russell said in a phone interview. "This part is noticeably different from everything else that the path of the wall does."

About 40 cenotes were included inside the city wall and would have served as vital sources of water in the semi-arid limestone plateau of the Yucatán Peninsula.

In fact, Mayapán was probably built in this location precisely because there were so many cenotes in the area. As part of their research last summer, Russell and his team identified 150 previously unknown cenotes around Mayapán, located during four days of aerial laser mapping.

Offerings to the Gods

View Images Photograph courtesy Bradley Russell, National Geographic Grantee

Before the archaeologists begin their dives, shaman Teodormio San Sores prepares for a traditional ceremony amid the steam from food offerings cooking on a fire.

In the modern Maya language, the ceremony is called jeets' lu'um (pronounced hets loom), literally "calming of the Earth."

Its purpose is to ask the gods for permission to enter the cenote, and to placate the cenote's legendary guardian—an enormous feathered serpent with a horse's head that the locals believe will snatch children who get too close.

The demon guardian is a combination of the feathered serpent of ancient Mexican myth and the steeds that the Spanish conquistadors brought to the New World.

Similarly, the ceremony combines beliefs from the ancient and colonial eras.

San Sores set up an altar on the table, oriented to the four cardinal points of the Maya universe and to the central axis that was thought to connect the Earth to the sky and the underworld. But he also placed a Christian cross on the altar.

His prayers addressed the old gods but included modern religious references as well.

He offered three foods to the east, west, north, and south: stewed chicken; a mixture of cornmeal, sugar, and water called saka'; and a thickened, spiced broth called k'ol.

He also left an offering at the cenote's entrance for the serpent—a gourd bowl filled with saka'.

When the divers finally penetrated deep into the cenote, their motions created a spooky parallel to the clouds of steam in this image. As they swam, they stirred up the limestone sediment, which then clouded the water.

At the same time, a limestone fog arose outside the cenote from a small fissure in the ground. "It was genuinely strange the first time we saw it," says Russell.

Taking a Dive

View Images Photograph courtesy Bradley Russell, National Geographic Grantee

Several days into the cave exploration, archaeologist Lisseth Pedroza Fuentes checks out a submerged rockfall filled with bones in the first chamber.

"From the very first dive, we had seen skulls," says Russell. "At the time of this dive, we were still doing an initial assessment and putting together a plan of action for mapping the cenote."

Flattened Skull

View Images Photograph courtesy Bradley Russell, National Geographic Grantee

An adult skull sitting upright amid the first chamber's rockfall debris likely belonged to a woman.

The top of the cranium was intentionally flattened during infancy. This method of deformation was widely practiced by the ancient Maya and is consistent with skulls that have been found in the customary burials from this same period.

"The hole in the forehead is probably postmortem," says Russell, "not the cause of death."

So far his team has identified ten skulls in this chamber, with more likely awaiting discovery among the rocks and sediment.

Skulls and Bones

View Images Photograph courtesy Bradley Russell, National Geographic Grantee

Skeletal remains scattered at the bottom of the rockfall slope include pieces of several human skulls and some long bones from modern cattle.

Drowned Cow

View Images Photograph courtesy Bradley Russell, National Geographic Grantee

A bovine skull lies upside down on the rocky debris in the first chamber.

"This must have been a cow that wasn't smart enough to avoid the hole in the ground," says Russell. "It's definitely not pre-Hispanic." If a cow takes a tumble into a cenote, there's no escape—it drowns, and eventually its bones drift to the bottom.

Herds of free-roaming cattle graze in the bush here, and ranchers use the area's cenotes like wells to water them.

Nice Teeth

View Images Photograph by Rait Kütt

The light from Russell's flash illuminates a brown, horseshoe-shaped lower jaw lying on the debris slope of the first chamber.

"All the molars have emerged, but the teeth are in beautiful condition—they weren't worn down over a long time," says Russell. "That means this is a young adult, about 18 years old."

The hard enamel of teeth often protects internal tissue, so Russell is hoping that this set will provide a good carbon-14 date for the cenote's human remains.

The Abyss

View Images Photograph courtesy Bradley Russell, National Geographic Grantee

At the bottom of the first chamber, Lisseth Pedroza Fuentes follows an intriguing tunnel that leads to a second chamber.

"This is very technical diving," says Russell. "You're crawling on your belly with your tanks against the ceiling." And there's no popping to the surface if something goes wrong.

The second chamber, completely filled with water, is about two times the size of the first. It has no opening to the sky and plunges to a depth of about 115 feet (35 meters). Part of its floor was also strewn with bones.

Aftermath of an Avalanche

View Images Photograph by Rait Kütt

Human remains in the second chamber include this partial skull that sits topsy-turvy amid rocks and sediment.

Russell believes that all the bones lay in the first chamber, originally. But at some point the floor collapsed, sending skulls, ribs, and femurs cascading down into the second chamber along with chunks of limestone.

Bare Head

View Images Photograph by Rait Kütt

In the second chamber, the top of another skull emerges from the sediment.

Russell's team spotted five skulls here. That brought the total from both chambers to 15, with indications that there may be more hidden in the debris.

Why does this cenote hold the remains of so many people, who appear to be males and females, teenagers and adults?

Most residents of Mayapán were buried under or near their houses, so this wasn't a normal cemetery.

The bones bear no marks that would indicate cause of death, so the people probably weren't sacrificed.

Other artifacts found here are mostly pieces of plain water pitchers, so there's nothing to indicate that these people were among the elite and getting some kind of special treatment.

Russell wonders whether the location of Sac Uayum is a clue.

For starters, it lies to the south of Mayapán, the direction that the Maya associated with the underworld—humankind's mythical place of origin, known as Xibalba (pronounced shee BALL bah). The dead might have been buried here to await the next cycle of creation.

Also, the builders of the city wall seem to have deliberately excluded the cenote from the city.

"Suppose these were plague victims," says Russell. "You wouldn't want them near the rest of the population. And you wouldn't want to drink the water either."

The evidence fits that theory, and so does the long-standing taboo.

Over time, the real reason to stay away from the cenote may have been forgotten, but the legend of the feathered, horse-headed serpent continued to keep people at a distance.

Older residents of the nearby village of Telchaquillo tell stories of people seeing the serpent perching in a tree, leaping up, spinning around three times, and diving into the water. But those sightings all happened once upon a time, as the stories go.

Younger villagers tend not to believe such tall tales.

At the Brink

View Images Photograph by Marilyn A. Masson

Perched on the limestone cap that partly covers the entrance to Sac Uayum, Russell contemplates the future.

He hopes to return to the cenote to continue mapping, and to excavate the debris slopes in both chambers.

He and his team have been working at Mayapán since the year 2000. "This was our best season ever," he says. "I don't know how we could top that."

But who knows? He could very well uncover more clues as to why people were once laid to rest in this watery grave.