Was This Mysterious Mountain Feature an Incan Tax Document?

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IIn southern Peru, Monte Sierpe or “Serpent Mountain” is dotted with more than 5,000 elusive, meticulously arranged holes, lined up for almost a kilometer. More than six centuries ago, these marks could have served as a tax spreadsheet for the Inca Empire, according to new findings published in the journal. Antiquity.

These mysterious mountain points were captured in aerial photographs by National geographic explorer Robert Shippee, published in 1933. Since then, they have captivated the public and researchers, who have proposed numerous explanations for their origins: fog capture, water harvesting, mining, gardening and even extraterrestrials, among other theories. Now a detailed analysis of this site, also known as the Band of Holes, has revealed intriguing patterns that suggest complex record-keeping, an international team of researchers says.

The Inca people appear to have used this feature continuously between 1400 and 1532, and it is near the intersection of once-bustling pre-Hispanic trade routes. It is also located between two Inca administrative centers: Tambo Colorado and Lima La Vieja. This suggests a link to commerce and government affairs, the authors of the new paper say.

Taking a closer look at Monte Sierpe via drone footage, they noticed that the holes are grouped into at least 60 sections. The researchers also spotted precise numerical patterns in these sections, such as an alternating repetition of the number of holes per row.

This reminded them of another Inca invention: khipusInca accounting tools made of knotted string that were used to tally information, including census data and tribute taxes. Researchers found more than 1,000 survivors khipus, and the one discovered near Monte Sierpe shows a similar organization of data into 80 groups of strings.

“The regularity of these squares is comparable to the numerical models of Monte Sierpe, suggesting a potentially similar purpose: counting and sorting different goods,” the paper notes.

These thousands of holes may have been turned into a spreadsheet after the Inca Empire took control of the region in the 15th century, which had previously been ruled by the powerful and sophisticated Chincha kingdom. As in other regions, the Incas introduced a tax system that required labor or tribute, such as products.

At Monte Sierpe, each section of holes may have been “linked to a particular social group for the payment of taxes and redistribution of goods,” the new article suggests. The range of numerical patterns they observed could be related to the khipus and 16th-century tribute lists in the Andes, and the size of taxpaying populations may have varied among specific towns and villages.

But this remarkable site may not have always functioned as a dynamic tax document. The scientists also studied sediment samples from holes throughout Monte Sierpe and discovered ancient pollen from plants such as willow, traditionally incorporated into baskets and mats, and corn, a staple crop in the Andes. Before the Inca Empire took power in the region, the Chincha kingdom could have used these holes to maintain a standardized barter system. “Dropping goods into the holes could have been a way to publicly display information about the quantity of goods available as well as the quantity of goods required for a fair exchange,” wrote study authors Jacob Bongers and Charles Stanish for The conversation. A specific number of holes filled with corn could have equaled a different number of holes filled with cotton, for example.

The authors say these theories are “tentative” but “bring us closer to the goal of one of the most enigmatic archaeological sites in the Andes.”

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Main photo by Charles Stanish

• Molly Glick

  Published on November 10, 2025

  Molly Glick is the newsletter editor of Nautilus.