MEXICO CITY—In 1519, Hernán Cortés and his conquistadors arrived in Cholula, one of the largest cities in central Mexico. Roughly 50 miles southeast from modern day Mexico City, its tens of thousands of residents sat in the shadows of the 17,000 foot Popocatépetl volcano. It had a temple featuring more stairs, claimed one Spaniard, than the main pyramid in Tenochtitlan. The Spanish tore it down, and rebuilt Cholula in the same fashion they did across Mexico—replacing “demon-worshipping” sites with Catholic ones.

That also meant a hermit’s shrine on top of a large hill called Tlachihualtepetl had to go. But the hill itself was in fact, no hill. Its name translates to “man-made mountain” and inside it was the largest pyramid remaining in the Americas, and by some estimates the largest monument ever constructed by man. But its secrets as one of the most important religious sites in Mesoamerica would remain hidden for 400 years—and is still being uncovered today.

“It’s the most difficult archaeological site in all of Mexico. Bar none.” That’s Patricia Plunket Nagoda, the American-born archaeologist and anthropologist who has spent the past few decades working with Mexican anthropologist Gabriela Uruñuela at the Great Pyramid to unlock its less than forthcoming secrets.