A hillside in Peru, featuring over 5,000 aligned holes, could have served as an immense Incan accounting tool or spreadsheet.

The “band of holes” that winds down the slopes of Monte Sierpe (Snake Mountain) has intrigued archaeologists since its aerial images surfaced in 1933. Various hypotheses have been presented regarding the purpose of these pits—ranging from tombs and defensive structures to water storage or horticultural uses during the Inca Empire, which flourished from 1438 to 1533.

“This 1.5-kilometer-long series of holes has stumped researchers for decades,” states Jacob Bongers from the University of Sydney.

To uncover the purpose of these holes, Bongers and colleagues examined sediment from 19 holes and utilized a drone to capture the clearest aerial visuals of sinkholes measuring 1 to 2 meters in diameter and between 50 centimeters to 1 meter deep.

The analysis unveiled pollen from consumable crops like corn, amaranth, chili peppers, and sweet potatoes, in addition to wild plants like sweet potatoes typhus rash (bush), commonly used for making baskets and rafts.

Bongers points out that the distance of the holes from fertile areas suggests pollen couldn’t have dispersed via wind. He proposes that local communities from the Chincha culture, active from about 900 to 1450 AD, may have lined the pits with vegetation and filled them with goods transported by llamas in woven baskets.

“Our data supports the theory that items were brought to this site and placed in the holes,” he says, noting that the absence of much pottery could be attributed to the use of baskets. “Initially, we believe this was a barter market, later evolving under the Inca Empire into a vast accounting device.”

Circa 1480, the Chincha fell under Incan dominance yet maintained autonomy; historical records indicate they were mandated to pay tributes, says Bongers. The concept of the accounting tool materialized from aerial photography, which enabled a more precise tally of the holes (approximately 5,200) and their arrangements.

The layout encompasses at least 60 sections or blocks. Researchers argue that it mirrors certain Incan counting methods, such as the knotted cord device known as Kipus, likened to a calculator or abacus. However, Bongers asserts that a more fitting analogy for this site’s design is a spreadsheet that documents food and goods contributions from the local populace.

“There are intriguing mathematical patterns; some sections consist of multiple rows of eight holes, with alternating counts of 8 and 7. This suggests deliberate intent behind their arrangement,” Bongers comments.

He believes the different sections may align with distinct groups from the densely populated and agriculturally productive regions surrounding Monte Sierpe, where approximately 100,000 people resided in the Pisco and Chincha valleys.

The specific Kipu structures from the Pisco Valley share layout similarities with the holes in the field, divided into roughly similar sections, consisting of a total of 80 departments.

“While the 5,200 holes are certainly large enough to accommodate items, they lack a clear decimal organization, which we would expect given the Incas’ decimal system; thus, the arrangement appears more complex than mere groups of ten,” remarks Karenlee Overmann from the University of Colorado Colorado Springs. “The hall itself is divided into approximately 60 sections, while the Kipu represents 80, indicating a notable numerical disparity.”

Bongers acknowledges this discrepancy but notes uncertainty around how long the site was in use or how the holes’ placement and function evolved in relation to the Kipu. “We are observing the end state, which might have started with just a few sections, growing in response to population increases,” he explains.

Located near a junction in the pre-Hispanic road network and between two major Incan administrative regions, Tambo Colorado and Lima la Vieja, supplies may have been collected here rather than in urban centers.

Overmann finds the study commendable for weighing various hypotheses about the holes’ purpose, but she suggests a simpler explanation could be at play. “Peru boasts a rich tradition of crafting substantial petroglyphs visible from long distances,” she mentions. “Perhaps they were simply engaging in that practice.”

Bongers concurs that this could be one of the functions. “However, both perspectives could coexist. It’s a massive, iconic form, yet it also fulfilled a practical role, serving as a type of social technology. Without the internet or cell phones, how did people know when and where to gather? Building a prominent site visible from afar may have been the solution.”

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