MEXICO CITY — In its heyday, Teotihuacan, central Mexico’s magnificent pre-Hispanic metropolis, was a wonder to behold. Laid out over 13 miles of fertile terrain, the city housed as many as 200,000 people. They came from all over ancient Mexico, flocking to the region’s first—and most impressive—urban center.

The people of Teotihuacan lived in their own “barrios”: neatly arranged residential complexes where they worked in a wide array of crafts. Dominating their skyline were the pyramids that have withstood the test of time, astounding visitors throughout the centuries. Mexican archaeologist Linda Manzanilla, the country’s foremost expert on Teotihuacan, likens the city to ancient Rome.

Yet despite its splendor, Teotihuacan has remained a mute, baffling mystery. The ancient Teotihuacanos did not leave behind any written records. Aside from painstaking forensic and archaeological analysis, we have no way of knowing who founded the city, what caused its decline and eventual demise around 450 AD, or even what the real name of the place was. (The name came from the Aztecs 700 years after it was abandoned: Teotihuacan, “the place where the gods were created.”)

The greatest mystery of all is who (what men or gods?) ruled the city, and how.

Manzanilla maintains that it wasn’t one single ruler or family that governed Teotihuacan. She thinks four “houses” ruled by committee.

“Mexican ‘Game of Thrones,’” I ventured on a recent visit.

She seemed perplexed. “Here we have a meticulously planned city, an exceptional city”, she said, shaking off my pop-culture gibberish. “But we don’t know who ran it. We need to find the rulers. And I think we’re close”.

Linda Manzanilla’s intuition might soon bear fruit if Sergio Gómez turns out to be right. For 12 years now Gómez—also a Mexican archaeologist—has been heading a showstopper of an archaeological dig: the underground tunnel that leads from La Ciudadela, a beautiful plaza at the city’s southernmost edge, to the Temple of the Feathered Serpent, arguably Teotihuacan’s most remarkable pyramid. Hidden for 18 centuries, the tunnel might just hold the answer to the city’s enigmatic past.

In the beginning, its discovery was pure luck. Gomez recalls walking into La Ciudadela on a chilly morning in 2003 and finding a small commotion. One of his assistants shared the news: a round hole, not more than 30 inches in diameter, had opened up 80 feet from the base of the Feathered Serpent. Without giving it much thought, Gomez, a thin, eloquent man with the rugged hands of a miner, asked his team to find a rope. They tied one end across his chest and lowered him, slowly, into the city’s depths. No one had been there for almost two millennia. Gomez descended at least 30 feet. He shone a light through the tiny cracks around him. “I had no idea what we had found,” he recalls today. “I couldn’t sleep for a week: I knew it could be a very important discovery.”

And so it was. After further studies, Gomez and his team concluded that what lay beneath their feet was indeed a tunnel. But not just any tunnel: a 330-foot passageway, 40 feet below the Ciudadela that led to a chamber directly underneath the geometric center of the adjacent temple. “An archeologist’s dream”, says Gomez.

After the initial excitement, Gomez and a small group of fellow explorers began the painstaking process of clearing the passageway. They used the most delicate of tools, removing inch after inch of moist and unstable earth. When an artifact turned up, the expedition came to a halt: it had to be dislodged, cleaned and cataloged before proceeding further down the tunnel. This happened often. The tunnel turned out to be a treasure trove like no other. After years of work, the group has found more than 70,000 pieces, including jade, ceramics, shells and various items made out of wood (before the tunnel, only one such wooden item had been found; the tunnel has yielded 4,000). The team has sifted through the mud with such care that they have even found—and duly labeled—insect wings.

All in all, Gomez and his peers have extracted 900 tons of dirt and relics from the tunnel. And they’ve done it all by hand, carrying bucket upon bucket on their tired backs, just like the Teotihuacanos did 1,800 years ago (there were no pack animals available for the construction of Mesoamerica’s Rome).

After a decade of work, the group finally made it to the last part of the tunnel, a place Gómez likes to describe as “a representation of the underworld.”

That’s where he took me last month, along with an eager crew of colleagues from Univision.

If the tunnel is an archeologist’s dream, it’s also a claustrophobe’s nightmare, damp and narrow. The descent requires a certain childlike balance along a set of planks capriciously placed along the floor. By now, Gomez and his men almost run through the place, like creatures in an underground horror movie. None of us had the same agility or courage.

Gomez stopped several times to help us with our footing but also to point out several of the tunnel’s most striking features. For example: when the Teotihuacanos finished building the tunnel, they closed it down. Walls were erected inside, placed every few feet along the way. Years passed. And then, Gomez says, they did something truly striking. After having blocked every possible entrance to their sacred passageway, the Teotihuacanos made their way in again. “They tore down their own walls to get in,” says Gomez: “We think they wanted to place something inside”. Then, the ancient inhabitants of the city rebuilt the inner walls and left the tunnel alone. It remained untouched until the 21st century.

What exactly did the Teotihuacanos place inside the most cavernous section of the tunnel, directly below the Temple of the Feathered Serpent?

Sergio Gomez has a theory.

Around 280 feet in, after a ceremonial antechamber that Gomez believes was used to cleanse and prepare for the final descent, the tunnel drops a further 10 feet. The entrance is slippery and narrow: the visitor has to crawl and claw. The area itself is so unstable that Gomez had to order special metal scaffolding to prevent a tragedy from happening.

The first thing I noticed was a long black line along the walls: telltale signs of condensation … perhaps water? Gomez confirmed it: for a long time, the last part of the tunnel had been under water, further proof of it representing the wet underworld.

Inside, after months of work, Gomez found an archaeological marvel: One, two and three lavish offerings, layer upon layer of beautifully carved, enormous conch shells (“Brought from the Caribbean all the way to Teotihuacan”); impressive ceramic pottery, shining in black; jade and other precious stones; shreds of ancient cloth. Everything neatly in place, stacked to protect something…or someone.

But the real prize awaited Gomez at the very end of the passageway.

There, the archaeologists uncovered four figurines, in their respective corners, all facing the center of the last chamber.

A few weeks after I left, Gomez and his team announced another astounding discovery: large amounts of liquid mercury near the area where the figurines had stood. ““It’s something that completely surprised us,” says Gomez.

Before we all crawled out of the place, before leaving the tunnel behind, I asked Sergio Gomez what he really thinks is down there. He echoed his colleague Linda Manzanilla. “It’s very likely that we are going to find tombs that belong to very important people, probably the city’s rulers,” he said with a childlike twinkle in his eye.

If Gomez indeed finds Teotihuacan’s rulers, if their faces emerge through the silvery mercury, the history of ancient Mexico will have to be rewritten. At last, Teotihuacan will have found a way to speak to us, from deep in its underground heart.