A small building in Mexico tells the violent history of treachery and conquest at the great Aztec city of Cholulu – but for hundreds of years, a secret lay beneath its floors.

Hiding under the grass, trees and soil sits the Great Pyramid of Cholula, deemed the largest monument ever built on Earth, with a base four times the size of the Great Pyramid of Giza.

In 1519, Hernan Cortez and his men marched into the city and massacred 10 percent of the population, building a tiny church on top of a massive hill as a symbol of their conquest.

And, it wasn’t until 1910 that the pyramid underneath was finally discovered.

In 1519, Hernan Cortez and his men marched into the great Aztec city of Cholulu, massacred 10 percent of the population and built a tiny church on top of a massive hill as a symbol of their conquest. However, hiding under the tufts of grass, trees and soil of this hill was the Great Pyramid of Cholula, which has a base four times the size of the Great Pyramid and is deemed the largest monument ever built on Earth

Cholula is located near the capital Tenochtitlan and was one of the most important cities in the Aztec Empire.

It was home to about 100,000 people and known for its bustling markets and fine trade goods.

The city was believed to be incredibly sacred and the people constructed a holy pyramid for every day of the year, reports Zaria Gorvett with BBC.com.

After the Spaniards settled in the city, which is now part of Mexico, they constructed so many buildings that it now has a church for every day of the year – the final touch being the Iglesia de Nuestra Señora de los Remediosa on top of the Great Pyramid of Cholula.

WHAT DOES HISTORY SAY HAPPENED IN 1516?

Cholula had an alliance of with the neighboring city Tenochtitlán, which they shared religious services and rituals with.

Upon the arrival of Hernan Cortez, Cholula was Mexico’s second-largest city, with a population of up to 100,000 by some estimates.

Upon the arrival of Hernan Cortez (pictured), Cholula was Mexico’s second-largest city, with a population of up to 100,000 by some estimates

It is believed that Cortez discovered the alliance between Cholula and Tenochtitlán, and became aware of a plot against the Spaniards.

Following the allegations, he decided to make a pre-emptive attack.

As a result, thousands were slaughtered by the Spaniards.

Cortez claimed 3,000 were killed in three hours, while others say the death toll was 30,000.

The massacre had the effect Cortez wanted and the people of Mexico were much more inclined to surrender.

Holy temples were destroyed and looted, and a church was built on the Great Pyramid of Cholula, which the Spaniards seemed to believe was only a large hill.

The city was believed to be incredibly sacred and the people constructed a holy pyramid for every day of the year, however, when the Spanish settled they created one church for every day of the year as a symbol of their Christian conquest – the final touch being the the Iglesia de Nuestra Señora de los Remediosa on top of the Great Pyramid of Cholula.

When Cortez, the leader of the Spanish Army, and his men arrived in Cholula on October 12, 1519, they stormed the streets, looted religious treasures, burned holy pyramids and killed 3,000 people within a few hours – although some say the death toll was as much as 30,000.  Pictured is an image of the church taken in the 1900s

The Great Pyramid of Cholula sits just outside Puebla, the fourth largest city in modern day Mexico.

It is 450 meters wide and 66 meters tall, and has been compared to the size of nine Olympic sized swimming pools.

Not only is this structure the largest pyramid on the planet, but it is also the largest monument built to this day.

The Great Pyramid of Chululo stands 450 meters wide and 66 meters tall, and has been compared to the size of nine Olympic sized swimming pools. Not only is this structure the largest pyramid on the planet, but it is also the largest monument ever built to this day

After the Spaniards settled in the city, which is now part of Mexico, they constructed so many buildings that it now has a church for every day of the year – the final touch being the the Iglesia de Nuestra Señora de los Remediosa on top of the Great Pyramid of Cholula

The locals call this massive formation Tlachihualtepetl, or ‘man-made mountain’, and because a church was built on top, it is also the oldest continuously occupied building in North America.

History suggests that when Cortez arrived in Cholula the pyramid was already thousands of years old and entirely overgrown by vegetation.

But some legends say the great pyramid was so sacred to the Cholula people, that they covered it with soil in order to hide it from the conquistadors.

The Great Pyramid has its own secrets, as it is not just a single pyramid, but no less than six built on top of the other.

The locals call this massive formation Tlachihualtepetl, or ‘man-made mountain’, and because a church was built on top, it is also the oldest continuously occupied building in North America. History suggests that when Cortez arrived in Cholula the pyramid was already thousands of years old and entirely overgrown by vegetation

Experts say it grew in stages, as successive civilizations improved on what had already been built.

‘They made a conscious effort to maintain and in some cases display previous construction episodes,’ says Carballo.

‘This is pretty novel, and shows deliberate efforts to link to the past’.

The pyramid hid under the earth until it was discovered in 1910, when workers were constructing an insane asylum at the base.

When pavement was removed in 2013 to access the city’s drainage system, other workers uncovered at least 63 skeletons – some of which were the deformed skulls of decapitated children

Cholula is located near the capital Tenochtitlan and was one of the most important cities in the Aztec Empire. It was home to about 100,000 people and known for its bustling markets and fine trade goods. The Great Pyramid of Cholula sits just outside Puebla (pictured), the fourth largest city in modern day Mexico

The pyramid hid under the earth until it was discovered in 1910, when workers were constructing an insane asylum at the base. When pavement was removed in 2013 to access the city’s drainage system, other workers uncovered at least 63 skeletons (pictured) – some of which were the deformed skulls of decapitated children

Although Tlachihualtepetl is the largest on the planet, experts are still trying to unravel its secrets.

It’s believed to have been constructed around 300 BC, but by who still remains a mystery.

Ancient myths tell a tale of a giant who built the structure, but the city’s inhabitants were a cosmopolitan mix and experts believe the mixed races played into the stories.

‘It appears to have been multi-ethnic, with a great deal of migration,’ David Carballo, an archaeologist at Boston University, Massachusetts, told BBC.com.

Although Tlachihualtepetl is the largest on the planet, experts are still trying to unravel its secrets. It’s believed to have been constructed around 300 BC, but by who still remains a mystery. From the 1930s until today, great efforts have been made to fully excavate the pyramid. Over five miles of tunnels have been dug inside the structure that are open to the public, however, the locals have also reclaimed the pyramid as their own

Cholula is located in the Mexican highlands and for thousands of years it was an important trading post, connecting the Tolteca-Chichimeca kingdoms in the North with the Maya in the South.

By the time Cortez set foot in the city, which he called ‘the most beautiful city outside Spain’, it had become the second-largest city in the Aztec empire.

From the 1930s until today, great efforts have been made to fully excavate the pyramid.

Over five miles of tunnels have been dug inside the structure that are open to the public, however, the locals have also reclaimed the pyramid as their own.