A series of about 5,200 holes stretching nearly a mile (1.5 kilometers) across the Pisco Valley in the southern Peruvian Andes has baffled researchers for nearly a century. But a fresh look at the site, called Monte Sierpe, or “serpent mountain,” may help archaeologists to decipher why ancient people constructed it hundreds of years ago.

The “band of holes,” as it’s informally called, first garnered attention when National Geographic published aerial photos of the site in 1933.

But there are no written records relating to the formation, leaving its purpose open to interpretation — and there have been many. Hypotheses about the holes’ use have included defense, accounting, storage, gardening, water collection, and fog capturing purposes. People who support ancient astronaut theory, a belief that aliens are real and shaped early civilizations on Earth, have also suggested extraterrestrial connections.

Now, new drone footage and a microbotanical analysis of pollen grains found within the holes are leading researchers to suggest that the site first served as a bustling marketplace for a pre-Inca civilization, and later as a method of accounting for the Incas, according to a study published November 10 in the journal Antiquity.

“Why would ancient peoples make over 5,000 holes in the foothills of southern Peru?” said lead study author Dr. Jacob Bongers, digital archaeologist at the University of Sydney and a Visiting Research Fellow at the Australian Museum Research Institute. “We don’t know why they are here, but we have produced some promising new data that yield important clues and support novel theories about the site’s use.”

Looking back in time

The sheer scale of Monte Sierpe has made it difficult to study, but drone technology has allowed for a new perspective, said study coauthor Charles Stanish, professor in the department of anthropology at the University of South Florida.

Each hole has a width of 3.3 to 6.6 feet (1 to 2 meters) and a depth of 1.6 to 3.3 feet (0.5 to 1 meter). The drone photography shows them to be organized into roughly 60 distinct sections that are separated by empty spaces, Bongers said. His team was also able to identify patterns — for example, a section of 12 rows that alternate between seven and eight holes, suggesting the organization is not random, he added.

Ancient pollen grains found inside the holes point to the presence of crops such as maize, as well as wild plants including reeds and willow that have traditionally been used to make baskets, Bongers noted.

Crops and other goods may have been placed in these baskets or deposited in the holes, which could have been lined with plant material. It’s possible that structures were also built over or near the holes, Bongers said, but there is no remaining evidence if any architecture ever existed.

The team believes that indigenous people of the pre-Inca Chincha Kingdom from Peru’s coast and highland may have come to exchange goods and barter using their own goods, rather than currency.

“Perhaps other important resources such as cotton, coca, maize, and chili peppers would have been placed in the holes and exchanged,” Bongers said. “For example, a certain number of holes containing maize would have been equivalent to a certain number of holes containing another type of good, such as cotton or coca.”

The clear evidence provided by the pollen helps rule out many of the other proposed uses of the site, said Dr. Dennis Ogburn, associate professor in the department of anthropology at the University of North Carolina at Charlotte. Ogburn did not participate in the new study.

“Monte Sierpe is a site that has truly been a mystery in Andean archaeology, and I am excited to see this research being done,” Ogburn wrote in an email.

Investigating a modified landscape

Dating of the holes suggests that the site was used between 600 and 700 years ago.

While the team is still carrying out more radiocarbon dating to determine an accurate timeline, they believe it was constructed during Peru’s Late Intermediate Period, between AD 1000 and 1400, which aligns with the pre-Inca civilization’s use of the site.

Pollen from citrus plants, introduced to the region during the colonial period between 1531 and 1825, suggests the site was still in use after the fall of the Inca Empire in 1532 as the Spanish colonized Peru. It was likely eventually abandoned “because the Spaniards could not find a way to integrate the site into their economic expansion,” Bongers said.

It’s possible that Monte Sierpe only included a few sections of holes as a barter marketplace before being expanded under the Inca empire.
Or, it may have been completed before the Incas arrived. But either way, the team believes the Incas used the site as a large-scale accounting device.

“In a sense, Monte Sierpe could have been an ‘Excel spreadsheet’ for the Inca Empire,” Bongers said.

The segmented organization of Monte Sierpe mirrors an Inca counting system involving knotted strings called a khipu. One of these devices, featuring 80 groups of cords, was recovered from the Pisco Valley.

“The numbers knotted on these cords show an intricate set of arithmetic interrelationships, suggesting that it is a surviving record” of the accounting operations that may have been used at Monte Sierpe, Bongers said.

Monte Sierpe was ideally situated for bartering and accounting near a pre-Hispanic network of roads, and between two major Inca sites called Tambo Colorado and Lima La Vieja.

Members of Bongers’ team continue to study khipus from Peru to test any potential numerical relationship between Monte Sierpe’s design and the Inca counting system. If there is a link, the holes may have been how the empire collected tribute, an early form of taxes, from local communities.

Given that the marketplace and accounting systems would have operated in two very different ways, and that the connection between the holes and Inca khipus is tenuous, more support is needed to be convincing, Ogburn said.

Preserving ancient heritage

As new research aims to unlock the secrets of Monte Sierpe, future findings could provide a glimpse into a piece of history that has not been well preserved.

“The Andes is one of the few world regions where ancient, large-scale societies, such as the Inca Empire, developed, but there is no strong evidence for pre-Hispanic currencies or writing systems,” Bongers said.

Dr. Christian Mader, a research group leader at the University of Bonn’s Center for Dependency and Slavery Studies in Germany, believes the paper is an important contribution to Andean archaeology, as well as the study of ancient economies. While Mader did not participate in this research, his work has focused on pre-Hispanic economies and exchange.

“Their proposal that the site served as a marketplace during the Late Intermediate period and as an accounting device for goods and tribute under the Inca Empire is interesting and convincing,” Mader wrote in an email. “And this paper illustrates how much we still have to learn about Indigenous economic mechanisms.”

Monte Sierpe presents a puzzle that is challenging to interpret, Bongers said, and part of the puzzle is presenting hypotheses, rather than arguments, that can be further tested to better understand local heritage.

“The narratives we build about local cultural heritage have real impact,” Bongers said. “It’s crucial to ensure that such narratives incorporate Indigenous perspectives and archaeological evidence to represent local heritage accurately.”

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