A magnificent pre-Hispanic metropolis once sat in central Mexico, spread out over 13 miles of fertile land and providing an impressive urban center for up to 200,000 people. Archeologists have called it Teotihuacan only because they don’t know its real name. In fact, there a lot of things they don’t know about the ancient city that apparently left no written records whatsoever and remains a historical enigma.While the site itself suggests grandeur and longevity, archeologists have struggled in vain to find an entrance to its secret world.

They do know, however, that ancient people apparently came from many areas around the year 450 to enjoy the splendor and sophistication of this great town and yet, informative artifacts have been surprisingly rare. Despite the mysteries (or perhaps because of them), researchers continue to see details about how the great city of Teotihuacan was ruled, structured, protected, and most importantly, destroyed. But just when scientists began to think the pursuit may be pointless, they made a discovery that not only illuminated their theories but gave them a direct path to finally understanding this archeological mystery.

Much like today’s modern cities, researchers knew the ancient people of Teotihuacan lived in “barrios” arranged neatly into residential complexes, where they must have worked, connected, and shared a wide array of crafts. The astounding pyramids of the city were the backdrop to their seemingly peaceful lives and still stand strong as a testament to their existence. Archaeologists who have been struggling to unveil the secrets of the lost city liken Teotihuacan’s communal structure to that of ancient Rome.

Although the city has been shrouded in mystery since the 1st century, one of the greatest curiosities for researchers is the lack of knowledge about how the city was ruled. It’s clear a great deal of time and thought was put into its structure, but who was in charge? Archeologists suggest it was something like a “Mexican Game of Thrones,” with four main “houses” ruled by a committee of people. Rather than having just one ruler, many educated theories suggest there were probably four different ones who shared the responsibilities of the bustling community. But this was mostly just a theory—until now.

For the past 13 years, Sergio Gómez, a Mexican archaeologist, has been searching for an entrance into the ancient city, which he has always suspected was buried deep below the earth. His project has continued to function on the premise that an underground tunnel exists from the beautiful plaza at the city’s southernmost edge to the Temple of the Feathered Serpent, the most remarkable pyramid. Hidden for 18 centuries, the tunnel would likely provide the answers to the city’s puzzling history. He would soon prove himself correct.

Arriving early to the dig site one morning, Gómez was greeted with the news that a small hole, about 30 inches in diameter, had opened up near one of the pyramids. Gómez immediately tied a rope around his waist and was lowered into what appeared to be the depths of an ancient city. Although it was still unclear what he had stumbled upon, he knew it was going to be a meaningful discovery.

What Gómez had, in fact, discovered was an archaeologist’s dream. His team had not only found the missing tunnel but had uncovered a 330-foot passageway, 40 feet below the city that led directly to the geometric center of the temple. Clearing the entrance to the tunnel alone took months because the soil was moist and unstable. When an artifact appeared during the process, as it often did, all work had to stop while the treasure was carefully dislodged, cleaned, and cataloged.

This process of literally unearthing the city took several years, eventually yielding more than 70,000 pieces, including jade, ceramics, shells, and wooden items. The team took such pains with the ancient findings, they even labeled insects they found in the mud.During this massive effort, Gómez and his team extracted 900 tons of dirt and relics from the tunnel. Just like the Teotihuacanos did 1,800 years ago, the researchers did it all by hand, carrying bucket after bucket of dirt out of the tunnel before moving forward. After a full decade of work, the group finally made it to the end of the tunnel, a place Gómez likes to call “a representation of the underworld.” The passage from this point onward became damp and narrow, descending steeply down towards the unknown.

Along the walls of the smaller passage, many striking images were seen on the walls that had been carefully erected inside. But one of the strangest things Gómez discovered was the fact that the tunnel was actually so deep it was underwater, and the walls had been built and then torn down and rebuilt at a later date. It seems the Teotihuacanos decided they needed to reenter the fully sealed tunnel after a few years, possibly to place something inside, and then seal it back up, where it had remained untouched until the 21st century. But what exactly did they place inside the most cavernous section of the tunnel? Once again, Gómez had a theory.

Along the walls of the smaller passage, many striking images were seen on the walls that had been carefully erected inside. But one of the strangest things Gómez discovered was the fact that the tunnel was actually so deep it was underwater, and the walls had been built and then torn down and rebuilt at a later date. It seems the Teotihuacanos decided they needed to reenter the fully sealed tunnel after a few years, possibly to place something inside, and then seal it back up, where it had remained untouched until the 21st century. But what exactly did they place inside the most cavernous section of the tunnel? Once again, Gómez had a theory.

Just when Gómez thought the dig could not produce anything more exciting, he found the real prize. At the very end of the tunnel, he came across four figurines in their respective corners, all facing the center of the final chamber. This seemed to confirm the original hypothesis of four separate but powerful rulers; could this really be the resting place of ancient kings? Quite possibly, yes. The final discovery of liquid mercury near the four figures, a material often associated with the underworld, suggests the existence of an important tomb or ritual chamber far below the ancient city, just waiting to be uncovered.

And the rest is history.