When Hernan Cortez and his Spanish army marched into Cholula in present-day Mexico nearly 500 years ago, they were greeted by a peaceful people prone to building pyramids instead of stockpiles of weapons.

Those people and their pyramids fell, and fast, with 10% of the local population murdered in a day as their pyramids were torched into oblivion. But as legend has it, one mud-brick pyramid was hidden, perhaps accidentally by vegetation, and was for centuries mistaken for a mountain, until locals began to construct an insane asylum in 1910.

That's when they discovered the largest monument ever constructed anywhere in the world. Tlachihualtepetl, or the Great Pyramid of Cholula, stands more than 200 feet tall and nearly 1,500 feet wide, dwarfing the Great Pyramid of Giza in volume, reports the BBC.

The Spaniards settled in Cholula and kept up with the local affinity for religious monuments, erecting enough churches so that there is now at least one for every day of the year.

But when they built the Iglesia de Nuestra Señora de los Remediosa on what they believed was a big hill, they were actually placing a sort of ornamental top on what is in reality a pyramid setup modeled similarly to Russian stacking dolls, this one stacked at least six pyramids high.

The original is thought to date back to around 300 BC, with each successive pyramid built over it by future civilizations. Today the "odd landmark" doesn't look like much more than a "grassy pre-Hispanic pyramid," as Afar magazine reports, but the marketplace that winds up from the pyramid's base to the church at its top is a testament to its exceptional endurance.

(Giza stands slightly askew.)

This article originally appeared on Newser: World's Largest Pyramid Was Mistaken as a Mountain