In Peru, the mystery of the 5,200 holes carved into rock is solved it was a pre-Inca economic system!

Fresh fieldwork and sky-high mapping now shift the debate from curiosity to clarity. What looked chaotic turns out to be ordered, practical, and surprisingly public.

A landscape-scale puzzle, finally mapped

Arcing for roughly 1.5 kilometers above the Pisco Valley, Monte Sierpe — often called the “Band of Holes” — consists of around 5,200 cavities, each about 1–2 meters wide and up to a meter deep. The strip measures about 19 meters across and breaks into distinct blocks separated by neat passageways. In some sections, low stone edging frames the pits; in others, holes are cut straight into soil and bedrock.

First spotted in 1933 in aerial photos by Robert Shippee, the feature sat largely unstudied for decades. Its remote setting shielded it from development, but also from urgent funding. Over time, speculation piled up: canals, granaries, gardens, tombs, or a ceremonial track. None truly stuck.

What the science reveals

An international team led by Dr Jacob Bongers (University of Sydney) and Prof Charles Stanish (University of South Florida) combined drone photogrammetry, high-precision GNSS mapping, and microbotanical testing of sediments taken from 19 pits across the site. The results, published in Antiquity, reshape the story.

Inside the pits, researchers identified pollen from maize and squashes, alongside wild plants such as Typha (cattail) and Salix (willow), plus diagnostic phytoliths. Maize pollen does not travel far on the wind. Its presence — absent in off-site control samples — suggests these plants came in with people, likely inside containers, not by chance.

Typha matters because Andean communities used it for basketry. The plant’s widespread traces at Monte Sierpe fit with goods arriving in woven baskets or bundles, being sorted, displayed, or temporarily stored in the pits before exchange or redistribution.

The mapping adds a second line of proof. The pits fall into rhythmic patterns. Some modules repeat with arithmetic regularity — for instance, nine rows of eight — separated by crosswalks that read like page lines. The geometry signals intentional grouping. It looks less like random digging and more like a ledger spread across the ground.

Why baskets matter

Woven carriers leave little behind. Threads rot. Wood breaks down. Microbotanical residues, though, cling to sediment. Typha signals basket use; maize and squash hint at the type of goods handled. In a dry coastal landscape, a pit can briefly cradle a bundle, then reset for the next transaction or count. It’s low-tech, quick, and visible to everyone present.

• Microbotanical residues inside pits point to intentional deposition of crops and plant materials.
• Regular sequences and blocks suggest counting units, not random holes.
• Crosswalks and aisles indicate planned circulation for people and goods.
• The location in a transitional ecozone favored coastal–highland exchange.

Chincha power and a market without walls

Between 1000 and 1400 CE, the Chincha kingdom dominated this coast. Historical work by María Rostworowski describes a major population with tribute obligations and a diverse economy: marine harvests, irrigated fields, fine craftwork, and long-distance trade. Chincha merchants sailed on balsa rafts and drove llama caravans into the highlands. Monte Sierpe sits in the chaupiyunga — the ecological hinge between Andean valleys and the Pacific littoral — a perfect staging ground for exchange.

The site layout fits a market that ran on equivalences rather than money. Goods arrive in bundles. Pits function as open bins and counters. People meet, compare, bargain, and move on. The open-air format means no permanent stalls, no roofs to repair, and no costly walls. The geometry does the heavy lifting.

Think of it as a market you can read at a glance: blocks for categories, rows for quantities, and paths for flow.

Folded into an Inca tax machine

In the fifteenth century, the Inca absorbed the Chincha realm. They rarely erased robust local systems; they repurposed them. Monte Sierpe appears to have shifted from a trade venue to an accounting ground embedded between two Inca administrative hubs: Tambo Colorado in the interior and Lima La Vieja nearer the coast.

Inca governance ran on a decimal hierarchy — groups of 10, 100, 1,000 — with labor levies (mit’a) and quotas in goods like maize, textiles, or dried fish. State record-keepers used khipus, knotted-string devices that captured quantities, categories, and places. The patterned blocks at Monte Sierpe echo the logic of such records, even matching sequences known from a khipu housed in Berlin, according to the research team. No clear colonial reuse appears in the record, suggesting the site’s prime administrative moment sat squarely in the late pre-Hispanic period.

How a khipu maps onto the ground

Picture a khipu cord with pendant strings. Each string carries knots that add up along simple base-10 rules. At Monte Sierpe, crosswalks behave like separators; pit blocks line up like pendant groups; repeated rows mimic knot tallies. Instead of knots on cotton, the “digits” become pits in soil. Administrators get an instant view: who brought what, which group met quota, and which fell short.

Why this changes the bigger picture

Monte Sierpe challenges the idea that complex accounting needs stone buildings or written ledgers. Here, a society turned landscape into infrastructure. The system relied on shared rules, not currency. It used visibility as governance: anyone could count pits, see shortfalls, and hold neighbors to account. The Inca then scaled that clarity into empire-wide administration without reinventing the wheel.

Architecture can be arithmetic. At Monte Sierpe, geometry stands in for paperwork and does it in full public view.

What to watch for next

Further sampling across more pit blocks could refine timelines: which modules are earliest, which show Inca-era reconfiguration, and whether seasonal use left distinct botanical fingerprints. Mapping wear patterns on crosswalks may reveal traffic intensity. If residue analysis picks up marine signatures near certain blocks, we might even see spatial clustering by commodity — fish here, maize there, textiles in another section.

Practical context and extra notes

Microbotany in a nutshell: pollen and phytoliths survive where wood and textiles fail. Maize pollen’s limited dispersal range helps distinguish human carriage from background wind. Typha’s presence is a proxy for basketry and transport, not just nearby wetlands. Control samples matter; the absence of these markers outside the strip strengthens the case for intentional deposition inside it.

A classroom or museum activity can model the system with beans and trays. Assign each tray a “community” and each row a “quota.” Move beans in bundles, count them in public, and mark when a group meets its number. It’s simple, fast, and oddly satisfying — and it captures why open counting fosters shared accountability.

Risks today include erosion, vehicle tracks, and souvenir picking. Low-cost stewardship — signage at approach points, scheduled local monitoring, and drone-based condition surveys — can protect the site without heavy-handed construction. For researchers, repeated high-resolution imaging after rare rain events may expose fresh microbotanical cues before they disperse.

Finally, a term to keep handy: chaupiyunga. It names that in-between ecological belt where coastal and highland networks intersect. Places on such boundaries are rarely quiet. They funnel goods, ideas, and power. Monte Sierpe thrived there for exactly that reason — a thin strip of pits turning a hillside into an economic instrument that both Chincha traders and Inca administrators could read at a glance.