The mystery surrounding the ancient city of Teotihuacan in Mexico continues to unfold after archaeologists found a hidden tunnel beneath the Pyramid of the Moon that they believe was built to represent the “passage to the underworld.”

Over 2000 years ago in 300 BC, Mesoamerican peoples began to develop larger settlements and built this great city that had once been home to more than 125,000 inhabitants, making it the sixth-largest city in the world at the time.

“It was the largest city anywhere in the Western Hemisphere before the 1400s,” Arizona State University archaeologist Robert Cowgill told National Geographic. “It had thousands of residential compounds and scores of pyramid-temples comparable to the largest pyramids of Egypt.”

Indeed, along the famed Avenue of the Dead one can find the Pyramid of the Sun, which is the largest structure in Teotihuacan and the Pyramid of the Moon, the second-largest located at the end of the road.

We don’t know who built the city exactly. The city predates the Aztec culture by 1,000 years, but there is evidence that many different peoples, including Mayans, lived in the city and influenced the architecture that has made it famous around the globe and now hosts millions of tourists every year.

Even the true name of the city remains unknown since it was the Aztecs who gave it the name we know today that means “birthplace of the gods”, while Mayan hieroglyphic texts identify it as puh, meaning “Place of Reeds.”

By the time the Aztecs moved in, the city had already been abandoned and lay in ruins, and archaeologists have been investigating Teotihuacan’s downfall for decades, offering several theories.

Some say the city was invaded and sacked by a foreign enemy. But there is evidence that an internal uprising caused by an ecological disaster resulted in a population decline and an overthrowing of the ruling class.

Because many structures showed evidence of being burned, researchers understandably concluded that a rival civilization attacked the city. The problem is that only structures belonging to the ruling class were burned, which indicates an internal revolt. But why did the population rise up against the ruling class?

It turns out that a famine during major droughts caused by a climate change in 535-536 AD could have been the cause. The city relied overwhelmingly on agriculture for food, growing everything from tomatoes, maize, beans, pumpkins and more, but once the droughts kicked in, the food supply started to dwindle and there was not enough food for the large population. A reason for this sudden climate change may have been the 535 AD eruption of the Ilopango volcano in El Salvador. After all, the volcano resulted in several Mayan cities being wiped out or abandoned, why not Teotihuacan?

Just because we don’t know exactly who built the city or why it was abandoned, it doesn’t mean we haven’t learned a lot about Teotihuacan and the people who lived here.

Archaeologists have been excavating the site for decades, and have found murals, stone masks, statues, figurines and evidence of human and animal sacrifices.

A lot of this evidence of human and animal sacrifice has been found at the Pyramid of the Moon, built between 100 and 450 AD at the end of the Avenue of the Dead. Connected to the street is a staircase that leads to a stage where these rituals were performed. The site of the pyramid also serves as a burial ground for the sacrificed and there is an altar dedicated to the Great Goddess of Teotihuacan.

In a recent excavation, archaeologists recently made an exciting find when they discovered a tunnel beneath the pyramid they believe served as a metaphorical passage to the underworld, which makes sense since humans were sacrificed at the site.

“In the explorations carried out at the end of the 1980s, through tunnels excavated in the body of the pyramid, archaeologists Ruben Cabrera and Saburo Sugiyama found skeletons of individuals with cranial deformation, as in the Mayan area, and various green stone objects, so it is not difficult to think that something similar could be found in the subsoil,” Dr. Veronica Ortega of Mexico’s National Institute of Anthropology and History told IFLScience.

“The tunnel is located to the south of the Plaza de la Luna,” Ortega continued. “But it is likely that there is another entrance to the east side, so it is essential to have complete radiography to know where its entrance is.”

Indeed, the reason why the team found the tunnel in the first place is that they used an imaging technique called electrical resistivity tomography to map structures below the surface.

Technology has been more crucial than ever before in archaeological excavations. New satellite imaging technology, for instance, has helped researchers identify thousands of Mayan structures and potential sites than had been previously known.

“These large complex offerings constitute the sacred core of the city of Teotihuacan, so all people considered it the mecca of civilization, hence what can be found inside can help unravel the relationships that this ancient metropolis with other regions of Mesoamerica,” Ortega concluded. “The discovery confirms that the inhabitants of Teotihuacan followed the same pattern in their large-scale temples and that their function was to emulate the underworld.”

It’s certainly an interesting development that makes us yearn for more knowledge about this ancient civilization and their great city. And perhaps archaeologists will find more as new technology is applied to the site. Perhaps one day, we will learn the true name of the city and find out who built it. For now, we’ll just have to settle for a creepy tunnel that we definitely would not want to enter at night.

Related: Abundant liquid mercury, mica, and pyrite found in tunnels beneath the pyramids

Featured Image: Pyramid of the Moon from Pyramid of the Sun, Teotihuacan by Daniel Case via Wikimedia (CC BY SA 3.0)