Colossal pyramid structures in the Americas as old as those in Egypt? The Sacred City of Caral-Supe, in central coastal Peru, boasts an impressive complex of ancient monumental architecture constructed around 2600 B.C., roughly the same time as the earliest Egyptian pyramid. Archaeologists consider Caral one of the largest and most complex urban centers built by the oldest known civilization in the Western Hemisphere.

The 1,500-acre site, situated 125 miles north of Lima and 14 miles from the Pacific coast, features six ancient pyramids, sunken circular plazas and giant staircases, all sitting on a windswept desert terrace overlooking the green floodplains of the winding Supe River. Its largest pyramid, also known as Pirámide Mayor, stands nearly 100 feet tall, with a base that covers an area spanning roughly four football fields. Radiocarbon dating on organic matter throughout the site has revealed it to be roughly between 4,000 to 5,000 years old, making its architecture as old—if not older—than the Step Pyramid of Saqqara, the oldest known pyramid in ancient Egypt.

That remarkable discovery places Caral as one of the oldest known cities in the Western Hemisphere. Coastal Peru has long been considered one of the six recognized cradles of world civilization, and new archaeological discoveries continue to push back the dates of when the region’s “mother culture” was established. Caral was the first extensively excavated site among some two dozen in a zone along Peru’s central coast known as the Norte Chico area. Archaeologists believe the sites collectively represent the oldest center of civilization in the Americas, one which lasted from roughly 3000 to 1800 B.C., completely uninfluenced by outside forces. It flourished nearly 4,000 years before the start of the powerful Incan Empire.

Despite Caral’s significance, decades passed between when the first scholars stumbled upon the area, and when they recognized its importance. German archaeologist Max Uhle explored the Supe Valley, but not Caral itself, as part of a wide-ranging study of ancient Peruvian cities in the early 1900s. But it was American historian Paul Kosok, who is largely recognized as the first scholar to recognize and visit what is now known as the site of Caral in 1948. In his 1965 book, Life, Land and Water in Ancient Peru, he referred to the site as Chupa Cigarro Grande, after a nearby hacienda_._

The sheer size and complexity of the site, however, led many to believe Caral’s structures were made only more recently, largely leaving it to go ignored. It was only in 1994, when Peruvian archaeologist Ruth Shady of National University of San Marcos started studying the site, that she realized, in the absence of finding any ceramics, that Caral might date before the advent of pot-firing technology.

But Shady needed to substantiate her claims. While excavating the largest pyramid, she and her team found the remains of reed-woven bags, known as shicras, filled with large stones to support the pyramid’s retaining walls. In 1999, she sent the reed samples for radiocarbon dating to veteran archaeologists Jonathan Haas, at Chicago’s Field Museum, and Winifred Creamer, at Northern Illinois University.

The results, published in the journal Science in April 2001, were monumental. Caral, along with other ancient Norte Chico sites in the Supe Valley were “the locus of some of the earliest population concentrations and corporate architecture in South America,” they wrote. At that time, Caral was hailed as the oldest known city in the hemisphere; since then, other Norte Chico sites, dating several hundred years earlier, have been unearthed—and discoveries continue.

Shady, who heads the Caral-Supe Special Archaeological Project, has continued to probe the depths of the long-lost ancient city—which, compared to some highly looted archaeological sites, had remained relatively untouched due to its late discovery. Her finds have revealed a site of epic proportions, one that not only looms large with its massive pyramids, but with other architecturally complex elements including extensive residential units, multiple plazas and an impressive sunken circular amphitheater that would have been large enough to hold hundreds of people. It was there that Shady and her team unearthed the remains of dozens of flutes made of pelican bones, relics of the importance of music to the ancient people of Caral.

Another significant find came in the Gallery Pyramid (or ​​Pirámide de la Galería), which reaches 60 feet tall. On the twelfth step of its central stairway, archaeologists unearthed a quipu, an ancient method of recording information used through Incan times, believed to be among the oldest discovered in the Americas.

That such a highly structured society existed implied the presence of strong leadership and thousands of manual laborers. “In order to set up this city and its monumental buildings according to a coordinated design, prior planning, experts and a centralized government were necessary,” wrote Shady in “The Sacred City of Caral,” a nomination file submitted to Unesco, which inscribed the archaeological site onto the World Heritage List in 2009.

The existence of such a large and well-organized inland civilization has helped smash the “maritime hypothesis,” or the long-running theory that complex Andean societies evolved first from the coast, where they were heavily reliant on fishing, and only later progressed inland. Dr. Shelia Pozorski, and her husband, Thomas, professors emeriti at the University of Texas Rio Grande Valley, questioned the maritime hypothesis as early as 1990. But it wasn’t until “archaeological evidence and dates for Caral and other Norte Chico sites became available, that the ‘maritime hypothesis’ was more soundly refuted,” she said. Coastal settlements and inland ones existed and developed at the same time, with Caral larger and more complex than any coastal villages that may have existed in the area before.

The discovery of cotton seeds, fibers and textiles, along with the remains of clam shells and fish bones at the site—as well as a large fishing net that was as old as Caral, found along the coast—helped Shady formulate another theory: The farmers at Caral grew and traded cotton in exchange for fish and shellfish from the villagers along the coast. The end result: production surpluses, local labor specialization and the emergence of political authorities in charge of trading. In short, Caral had a burgeoning model of social and political organization.

While much is known about Caral, perhaps there’s more that’s unknown—including why it developed in such a complex manner, and its relationship to the other ancient Norte Chico sites.

The largest mystery, however, might be why it was abandoned after being inhabited for nearly a thousand years. “There is no concrete evidence indicating that a single event, like an earthquake or large flood, ended the occupation of Caral,” Dr. Thomas Pozorski says. “It is much more likely that several factors contributed to the decline of the state, including internal strife and discord which is very difficult to prove in archaeology.”