© Meghan DhaliwalRamiro Ruiz, a caretaker of the archaeological grounds of San Pablo Villa de Mitla in southern Mexico, descends into a tomb belonging to the site’s ancient Zapotec ruins.
The ruins of Mitla sit about 30 miles from Oaxaca in the mountains of southern Mexico, built on a high valley floor as a gateway between the world of the living and the dead. The site was established in roughly 200 A.D. as a fortified village, and then as a burial ground by the Zapotecs, the so-called Cloud People, who settled in the region around 1,500 B.C.

Comment: That last year seems to be a typo.

Five main sets of ruins are scattered throughout the small modern tourist hub that is San Pablo Villa de Mitla. Some are royal houses and ceremonial centers featuring central plazas. One is a crumbling pyramid, and another is a domed Spanish church with adjoining Zapotec courtyards. Elaborate mosaics cover the walls, meandering geometric friezes resembling carved lace; "petrified weaving" is how Aldous Huxley described them in his 1934 travelogue, "Beyond the Mexique Bay." Traces of color linger on masonry that was once slathered in bright red paint made by grinding cochinillas, wood lice that live on nopal cactuses.

Spanish chroniclers christened Mitla the Vatican of the Zapotec religion, and its wonders were said to continue underground. The Zapotecs, known for their metaphysical connection to rain, thunder and lightning, believed that they could commune with gods and ancestral spirits in an earthen cavity below their city, which led to a netherworld known as Lyobaa, the "place of rest."

In 1674, Francisco de Burgoa, a Dominican friar, wrote an account, based largely on church documents, of Spanish missionaries who had explored a sprawling labyrinth of tunnels and burial chambers beneath the ruins of a monumental palace. A century earlier, secular clergy had blocked the doorways to the sunken complex with bricks and mortar, presumably either to keep the masses out or the ghosts in.

"The Spanish believed that demons performed black magic in the underground tombs," said Denisse Argote, a researcher at the National Institute of History and Anthropology in Mexico. In September, Dr. Argote and a team of 13 geophysicists, engineers and archaeologists spent a week at Mitla for the second season of an ambitious exploration to determine what remains of the Zapotec's long-abandoned catacombs. In the still, steady calm of morning, they lugged around enough electronic ganglia to jump-start the Bride of Frankenstein.

Hard by the courtyards was the Church of San Pablo, a Catholic house of worship. The church was built in 1590, seven decades after Spanish conquistadors arrived in the Oaxaca Valley. Members of the Dominican order assembled it atop the sacred ruins, repurposing stones from the palace. By severing the Zapotecs from their pagan deities, the missionaries hoped to convert them to Christianity. "Instead of trying to kill off the Zapotec's religious beliefs," Dr. Argote said, "it was easier just to wash their brains with new beliefs."

The researchers were denied permission to set up equipment inside the church, so they had placed seismic sensors — electrodes and geophones — in a horseshoe arrangement on the patio, to peer down through layers of soil. By early afternoon a tangle of cables covered the yard like mangrove roots.

"We use noninvasive geophysical survey tools, to eliminate the need to dig into the bedrock and disturb the site," said Marco Vigato, the research team's prime sponsor. "Our hope is to detect hidden spaces and buried objects or other evidence of the lost subterranean chambers described by Father Burgoa."

Last year the researchers used a combination of three scanning technologies — ground-penetrating radar, electrical resistivity tomography and seismic noise tomography — to generate three-dimensional images of what lay below. The surveys revealed a mysterious underworld, confirming the presence of a large void under the sacristy that extended to the west and northwest.

The team also identified two east-west tunnels, varying in depth from 16 to 26 feet, entering the cavity from an easterly direction. "This corroborates what is described in the historical records of the Mitla area and the sayings by the Mitla people," said one crew member, Andrés Tejero-Andrade, an engineering professor at the National Autonomous University of Mexico.

At one point in the afternoon, Mr. Vigato stood in the church's gilded sanctuary. The sound of children playing filtered in through the windows; in front of him was the main altar, draped in white cloth. Directly below it, hidden from view, was a sealed-off portal that he had determined was the entrance to the underground labyrinth.

"I believe we have found the lost palace of the living and the dead," he said.

From eternity to here

Tall and angular, Mr. Vigato talks about the Zapotec culture with the boyish enthusiasm of an Eagle Scout describing his merit badges. He grew up comfortably in Milan and traveled widely with his parents. He attended Harvard Business School, during which time he met Daniela Thions Meyer, a native of Mexico who was getting her M.B.A. at Dartmouth College in New Hampshire. They married and eventually moved to Mexico City, where he became a Walmart executive for a while. His fortune comes largely from his family, and from his work in retail and as a strategy consultant.

In his spare time he wrote "The Empires of Atlantis," an alternative history of the origins of civilization in which he proposed that "Atlantis, not Africa, was the true cradle of mankind." The theory elicits a chuckle from Dr. Argote, the National Institute researcher. "I will only believe in lost civilizations when I see material remains of them," she said. "In the Mitla project, we are only using scientific methodologies whose applicability and success have been proven in previous research and supported by peer-reviewed scientific publications."

Several years ago while reading about the folklore of Mitla, Mr. Vigato began entertaining the possibility of confirming Father Burgoa's account by probing the subsoil. In 2021, he founded the Archaeology Research and Exploration Project, or ARX, to raise money for archaeological research. The nonprofit organization has partnered with the National Institute of Anthropology and History and Mexico's secretary of culture to document and conserve San Miguel Ixtapan, a site in the town of Tejupilco that has yielded dozens of carved megalithic stone slabs of unknown age and origin. The ARX excavation has unearthed two additional slabs and created 3-D models of the "Maqueta" stone, an elaborate, centuries-old representation of a city sculpted in a huge basalt boulder.

The door to Lyobaa

According to Father Burgoa's account, Mitla's underground ossuary had four connecting chambers of diminishing size whose dimensions mirrored those of the palace and courtyards above. The first room was a shrine filled with totems in which high priests, dressed in white robes with embroidered chasuble, issued oracles and funeral rites in a cloud of perfumed copal incense.

"When they were to sacrifice human beings, the ceremonies were more elaborate," Father Burgoa wrote in Spanish. "Their ministers would lay the victim on a large slab, exposing his chest. With flint knives, they would tear the chest open, amid the victim's horrifying convulsions, revealing the heart, which was torn out along with the soul, which the demon took. The heart would be given to the high priest, who would then offer it to the idols."

The next room contained the tombs of high priests; the third held a crypt for kings, whose mummified bodies were dressed in military attire with spear and shield. At the end of the fourth room, Father Burgoa wrote, was a stone door that opened to Lyobaa, a yawning cavern honeycombed by passageways, its roof constructed of immense stone slabs supported by stone pillars 15 feet high. The cave, he wrote, was 30 leagues deep, the equivalent of 78 miles.

The friar also detailed a form of "living" sacrifice in which the victim was tossed, sometimes willingly, into the dark hollow and left to die of thirst and starvation. Javier Urcid, an archaeologist at Brandeis University, called Father Burgoa a great embellisher. "Oracular divination and human sacrifice were integral ritual aspects of royal houses, but letting penitents inside a supposed cave and then sealing its entrance is far-fetched," he said.

Aside from the church site, Mitla has four distinct groups of buildings, two of which were excavated and fully restored by the 1980s, and all of which are suspected of concealing a web of underground corridors. The most famous and best preserved ancient monument is the Palace of the Columns, the former living quarters of the high priest, whom Father Burgoa likened to the pope. The high priest's authority was even greater than that of the Zapotec king. He presided over human sacrifices staged behind the Column of Death, a thick pillar that cleaves the entryway to a lithic tomb.

Legend has it that if you hug the pillar, the space between your fingertips will foretell the number of years you have left to live. Longevity is in inverse proportion to arm length. An oft-told tale involves a long-armed out-of-towner who was surprised to learn that, according to the Mitla oracle, he had already been dead for several years.

Tunnel to somewhere

During this year's expedition, researchers pushed georadars across the fields and plazas of Mitla like souped-up lawn mowers. To supplement the radar arrays, the team added magnetometry, a technique that allowed it to detect geomagnetic anomalies that may signal the presence of buried objects or cavities, natural or artificial.

Scans and soundings of the ground below one group of buildings showed extensive terracing and retaining walls. Mapping of the area beneath the pyramid turned up an irregularity that looked to be a tunnel. There were no signs of passageways under the Palace of Columns, but the team did identify a stairway that led down to a pair of doors and, at a greater depth, chambers that could be tombs.

Contrary to Mr. Vigato's declaration, it was unclear whether the team had located an entrance to the labyrinth beneath the church. Ideally, he said, they would probe the anomalies with an endoscopic camera. "This would cause only minimal disturbance to the site, requiring the drilling of a hole no more than a few centimeters wide, which will be immediately filled afterward," Mr. Vigato said. The images would provide precious information on the nature of the cavities and their possible contents.

But the church committee, an elected body largely independent from church hierarchy and the diocese of Oaxaca, has repeatedly turned down requests to conduct more research around that portion of the site. Its opposition appears to be motivated by a fear that the church would be razed to allow for archaeological digs, and that any riches found would be requisitioned.

"All these concerns are unfounded," Dr. Argote said. "By federal law, no one can demolish an historical monument to recover pre-Hispanic remains. And any so-called treasures would mostly consist of ceramics, human remains or lithic artifacts, not gold or jewels as the locals believe."

The latest geophysical data indicates that the tunnel or chamber beneath the church continues farther west and farther down. What the researchers don't know, and may never know, is how far it continues and how deep it goes.

Venturing into the void

In the Zapotec worldview, caves are living beings. The mountainous landscape of the Oaxaca Valley conceals scores of grottoes that during more than 10,000 years of human occupation have served as sanctuaries associated with deities and forces of nature that can be coaxed, flattered or angered by mortals. Among the modern inhabitants of Mitla, rumors about the mazelike cavern seem to have never died out.

The ossuary remained inaccessible from the late 16th century until the middle of the 20th, when the Church of San Pablo underwent a major renovation and cupolas were added to replace the original wooden roof. "The entrance under the main altar was unblocked and some kind of door was installed," Dr. Argote said.

Evidently, the portal to eternity remained an open secret at least through 1992, when a couple of neighborhood boys, Omar Santiago Garcia, 11, and his brother, Eder, 9, got jobs sweeping the floors. One day the janitor asked if they would like to sneak a peek at the cave where the dead souls lived. The boys nodded apprehensively.

"Be quick," the janitor cautioned. "And don't go far or you will never find your way out."

He reached under the altar and moved the planks that concealed the entrance. The boys gazed into the void. "I was the brave one," Omar Santiago, now 33, recalled in September. "I took the first step."

Lacking a flashlight, they navigated the narrow stairway, slowly, carefully, holding onto the decorative friezes that studded the stone walls. At the bottom of the stairs, they panicked. "I felt I was trespassing where the living should not be," Mr. Santiago said. He and his brother scrambled up the steps and didn't look back. Eight years later the entrance was capped with a concrete slab, and, for good measure, sheathed in floor tile.

Mr. Santiago related all this from a seat in Restaurante Yainadoo, the taqueria he runs across the street from the church. He still remembers when his mother told him about a man who ventured into the maze and strayed into a roomful of people having a fiesta. When a spooky-looking woman offered the man a tamale, he turned around and fled. "The man later described the woman to his wife," Mr. Santiago said. "His wife was shocked: The woman had died many years before."

There was the slightest of pauses.

"My mother said, 'Remember, Omar: Never accept tamales from dead people.'"