Archaeologists, quite rightly, love a secret and mysterious tunnel. Whether they’re concealing a cache of mummies or they’re designed to mimic the plumbing network of an artificial volcano, they never fail to stoke the fires of the imagination – and, of course, these tunnels always lead to some sort of treasure.

Now, a brand-new passageway has been identified by researchers at the National Institute of Anthropology and History (INAH) in Mexico City. Using CT scans, a lengthy hidden tunnel has been found 10 meters (roughly 33 feet) beneath the surface of Teotihuacan, a truly ancient metropolis just north of the modern capital.

The tunnel was found under the grandiose Pyramid of the Moon, and it extends towards the central square – an area used for human sacrifices and other long-lost rituals. This pyramid is the second largest structure in the city, and it’s surrounded by a smaller version of it, where the public were thought to stand and watch the sacrifices take place.

The team at INAH believe that the tunnel resembles that found in other sites across the region, which were often covered with offerings and were designed to imitate passages to the underworld. The origins of life would have been emulated in the tunnel’s features in some way or another.

There’s also a chance that it’s the central spine of a network of concealed tunnels that snake their way beneath the surface.

The pyramid itself, built in part to honor the creator, the Great Goddess of Teotihuacan, is full of human sacrifices and people buried alive, along with shards of obsidian, greenstone, and various animal skeletons. As the tunnel has yet to be excavated, the researchers are unsure what exactly it may contain, but it’s likely similar objects and corpses will be found.

The origin of the founders of Teotihuacan remains mysterious, but it’s likely it began to be built between the 1st and 7th centuries. By the time the first millennium ACE rolled around, it was the largest city in the pre-Columbian Americas, containing around 125,000-200,000 residents.

It’s unclear if it formed the center of an empire or if it was more of a city-state, but either way its architectural and cultural influences could be seen across Central America. It came to be adorned by thousands of murals whose beauty have been compared to that of other masterful styles across the world.

At one point, around the time the influence of Teotihuacan began to spread across the region, the city was seemingly ruled by a man whom, based on the descriptions of him by the Maya, archaeologists have dubbed “Spearthrower Owl”.

There was clearly no threat to its existence for some time, as no military garrisons or fortifications of any kind could be found around the site. Eventually though, as always seems to be the case, the civilization collapsed and the city fell into ruins.

Archaeological evidence reveals that it was sacked and burned to the ground around the year 550 ACE. As most of the damage was done to buildings housing the ruling class, it’s likely the sacking was induced by the plebiscite.

The Aztecs happened upon it in the 14th Century, claimed a shared ancestry, and took it for their own. They were the ones that gave the temples their commonly used names, including the Pyramid of the Moon. They likely never knew about the tunnels hiding underneath it.

Eventually, after the transformation of the country via colonization, conquest, and globalization, it became a UNESCO World Heritage Site, and now researchers are spending their days uncovering as many secrets from its storied past as possible.