In Mexico sits the Great Pyramid of Cholula, also known as the Great Pyramid of Tepanapa, a 2,000-year-old human-made structure that went completely unnoticed by the Spanish army when they invaded in 1519. An impressive oversight when you consider that it’s the biggest pyramid on Earth. How did they miss it? Quite simply, it’s hidden inside a hill.

By RACHAEL FUNNELL

The Cholula Massacre was one of the most infamous endeavors of conquistador Hernán Cortés who stormed the city of Cholula on October 12, 1519. Cortés’s army arrived anticipating a fight but were met with a sacred city believed to be home to a holy pyramid for every day of the year; structures built to place divine protection over the city.

Instead, as the lives of 3,000 people were taken (amounting to 10 percent of the city’s population) the many pyramids became the subjects of looting. Having claimed the many religious artifacts, the Spanish settled in Cholula and began erecting their own churches.

One of those, the Iglesia de Nuestra Señora de los Remediosa, was to become – effectively – a hat, after it was unknowingly placed on top of the Great Pyramid of Cholula. At 66 meters (216 feet) tall and 450 meters (1,475 feet) wide, it’s the largest pyramidin the world, with a volume that’s nearly twice that of the Great Pyramid of Giza.

Known as Tlachihualtepetl ("man-made mountain”), the pyramid actually boasts the longest occupation by humans of any building, helped along by its Spanish church bonnet. A lofty claim for the largest monument ever constructed by humans in Mesoamerica.

It was built in four major construction stages, each one stylistically unique but built without concealing the constructions of the past, which some expect was a purposeful move to preserve its history. It was an important shrine dedicated to mountain worship and a rain deity until the Spanish Conquest.

“In Cholula there was a man-made hill called Tlachihualtepetl,” archaeologist and authorGeoffrey McCafferty reports Durán Diego wrote in The Book of the Gods and Rites and the Ancient Calendar [1576 – 1579]. “It was called thus because it is said that the Giants built it in order to climb up to the heavens; today it stands in ruins. This hill was much hallowed; there were the usual and unceasing adoration, the prayers, the great sacrifices, offerings, and slaying of men.”

As for how the enormous pyramid came to be buried, we don’t yet know for absolute certain. One theory is that the dizzying structure was lost to foliage after an extended period of disuse, perhaps due to abandonment as a result of volcanic activity or flooding. Another is that it was buried on purpose by the Aztecs in preparation for the conquistador’s arrival.

One way or the other, by the time Cortés and his men arrived, the world’s largest pyramid had morphed into a convincing hill and, evidently, one fit for a church. La Iglesia de Nuestra Señora de los Remedios still sits atop the Tlachihualtepetl to this day.

However, it would not go unnoticed for long.

“The archaeological significance of the pyramid was quickly discovered, however, when ‘excavations’ in 1535 by the Franciscan priest Toribio de Motolinia recovered ‘idols’ and conch shell trumpets from the summit of the Great Pyramid while erecting a cross,” wrote McCafferty.

By the turn of the 19th century, the German explorer Alexander von Humboldt began archaeological investigations at the site. He called the pyramid “the greatest, most ancient, and most celebrated of the whole of the pyramidal monuments” in Mexico. By the 1930s, further investigations began to chip away at its exterior.

In the 1970s, a series of tunnels stretching 8 kilometers (5 miles) in total gave further access to the pyramid’s interior and many layers. McCafferty estimates there are four construction phases contained within it, not including smaller extensions and buildings that were eventually swallowed up by later stages of Tlachihualtepetl’s construction.

Construction styles included "adobe" bricks made of mud that were extremely resilient in region’s climate, while other bricks had a veneer of mortar, and some were finished with plaster. The pyramid’s placement also seems to have been carefully selected in the context of its environment, built over a natural spring with an interior chamber which may have acted as an artificial “cave” to be symbolic of a portal to the underworld.

Its cosmological orientation at 24-26° north of west may have also tapped into the summer solstice, explains McCafferty, with the pyramid’s western staircase facing the sunset as the temple at its peak took in the last light on the longest day of the year. We know little of its creation, however, so it’s difficult to know how intentional these decisions were and if they were indeed incorporated into the structure’s spiritual practice (but it sure sounds like golden hour on steroids).

The innermost structure of the stacking dolls of pyramids was a square pyramid which stood at about 17 meters (56 foot) tall. Inside, preserved paintings were discovered that depicted an insect-like body with a skeletal head captured in vivid colors, “perhaps a butterfly in the process of meta morphosis,” suggested McCafferty.

This was encased by a second pyramid which stretched to a heigh of 35 meters (115 feet) with steps across all its sides making it accessible from any direction. Several alterations were built upon this, bringing the pyramids total height up to 66 meters (217 feet) when stage three began, extending its base dimensions to 350 meters (1,150 feet) each side. By now, the pyramid was made up of enormous platforms arranged around a central peak.

Notable features later discovered of the pyramid and its many extensions include the Patio Of The Carved Skulls, so named from the plaster-covered human skulls which were built into its altar. Inside that alter were the remains of a male and female alongside grave goods, which McCafferty places among the “richest” burials known from Cholula.

There remains much to be learned about the world’s largest pyramid, but the good news is McCafferty has received funding to continue explorations and investigations at the site, with his website suggesting he returned to Cholula last year. While IFLScience was unable to reach out for comment, we eagerly await his findings.

As for visiting the Great Pyramid of Cholula, tourists are welcome at the historic site, which is also home to the UNESCO-listed historic center. Tours can be arranged from Mexico City and will enable to you explore the Cholula ruins and its underground tunnels.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.

[H/T: Atlas Obscura]