Archaeologists led by Jacob Bongers report new evidence about Monte Sierpe, a 1.5 km ridge in southern Peru cut with roughly 5,200 pits. Microbotanical analysis of sediments from 19 hollows recovered maize, Amaranthaceae (including quinoa), Pooideae cereals, Cucurbita and basket materials, suggesting baskets of food were placed in the pits. A charcoal date of 1320–1405 CE and surface pottery point to pre-Inca Chincha activity, while aerial patterns resembling khipu indicate the Inca later may have repurposed the site as a tribute or accounting register. Researchers plan further sampling and dating to test this market-then-accounting model.
Why Peru's 5,200 Monte Sierpe Holes May Have Been a Marketplace — Then an Inca Accounting System
Archaeologists led by Jacob Bongers report new evidence about Monte Sierpe, a 1.5 km ridge in southern Peru cut with roughly 5,200 pits. Microbotanical analysis of sediments from 19 hollows recovered maize, Amaranthaceae (including quinoa), Pooideae cereals, Cucurbita and basket materials, suggesting baskets of food were placed in the pits. A charcoal date of 1320–1405 CE and surface pottery point to pre-Inca Chincha activity, while aerial patterns resembling khipu indicate the Inca later may have repurposed the site as a tribute or accounting register. Researchers plan further sampling and dating to test this market-then-accounting model.
12:58 PM, 11/10/2025Science
Who carved 5,200 holes into an Andean ridge — and why?
For centuries a long ridge in the southern Peruvian foothills has borne a baffling pattern of man-made hollows. Known as Monte Sierpe, the feature first drew wide attention after aerial photographs published in 1933. New fieldwork led by Jacob Bongers of the University of Sydney offers fresh evidence that helps explain the site's complex history.
The earthwork is a substantial engineering project: a strip roughly 1.5 kilometers long and seven to eight hollows wide, containing some 5,200 excavated pits. Many pits were cut into the sediment and several were deliberately lined or reinforced with stone. The scale of construction implies careful planning and considerable labor, which helps explain the decades of speculation — past proposals included gardens, water catchment or fog collection systems.
Bongers and colleagues performed extensive drone mapping and systematic sediment sampling from 19 pits. Microscopic analysis recovered starches and pollen consistent with maize (corn), members of the Amaranthaceae family (which includes quinoa), cereal grains from the Pooideae grass subfamily (such as oats, wheat and barley), and Cucurbita (squash). The team also identified bulrush and willow remains typical of basketry materials. These finds suggest baskets of foodstuffs were placed into the hollows at some point in the site's use history.
Radiocarbon dating of charcoal from one pit returned a range of about 1320–1405 CE, indicating that at least some deposits predate Inca expansion into the valley around 1400 CE. Surface pottery recovered near the site is consistent with occupation by the earlier Chincha culture, who lived in the region for centuries before Inca incorporation.
Based on the botanical evidence and population estimates for the broader valley, the authors propose that Monte Sierpe may originally have functioned as an open exchange hub or marketplace, where mobile traders, llama caravans, farmers and fisherfolk deposited goods in baskets for trade. Aerial imagery adds a second layer to the interpretation: the pits are arranged in grouped blocks whose overall pattern resembles an Inca khipu, the knotted-string devices used for accounting and record-keeping.
"Fundamentally, I view these holes as a type of social technology that brought people together, and later became a large-scale accounting system under the Inca Empire," says Jacob Bongers.
The combined evidence supports a two-stage model: first a local pre-Inca marketplace or exchange system, and later repurposing by the Inca as a landscape-scale tribute or accounting register — a sort of "landscape khipu." The authors emphasize that questions remain, for example why such a monument appears limited to this valley rather than widespread across the Andes.
Next steps include a second field phase to sample additional pits, obtain more radiocarbon dates, and compare the site's organization with local khipu examples to test and refine the marketplace-then-accounting hypothesis. The study appears in the journal Antiquity and adds a compelling new chapter to our understanding of how pre-Hispanic Andean communities organized exchange, storage and record-keeping across landscapes.