A study published in Nature on July 29, 2026 found more than 20,000 ancient structures hidden across the Southwest Amazon. The structures, called earthworks, were built by Indigenous peoples roughly 2,500 years ago. Most of them are geoglyphs — large designs dug into the ground that form geometric shapes or figures, best seen from above.

This is one of the biggest archaeological surveys of ancient Amazonian earthworks ever done. The 20,000-plus number reflects what the research team was able to find and catalog, and most of the structures fall into the geoglyph category (Science).

The team used radiocarbon dating to figure out how old the structures are. Radiocarbon dating works by measuring carbon — a building block of all living things — that slowly breaks down after an organism dies. By measuring how much has broken down, scientists can estimate age. The results placed the earthworks within a 2,500-year range. The study does not claim all the structures were built at the same time; instead, the dating shows a range across which precolonial peoples built and used them (Nature).

The Southwest Amazon has produced evidence of earthworks before, but the sheer number in this study — over 20,000 — greatly expands what was known. Geoglyphs in this region typically consist of ditches and raised banks forming geometric patterns, though the study's classification covers the variety of earthwork types the team found.

The broader context here is a shift in how scientists understand the Amazon's past. For decades, the leading idea was that the Amazon's poor soil and thick forest could not support large, settled populations with complex societies. That idea has been challenged by a growing body of evidence: pockets of human-made fertile soil, raised farming fields, ancient roadways, and now earthworks on this scale. The 20,000-plus figure adds weight to the argument that precolonial Amazonia was home to larger and more organized populations than experts once thought.

What remains uncertain is what these geoglyphs were for. Researchers have proposed a range of purposes — ceremonies, defense, housing, or water management — but there is no consensus. The Nature study documents and dates the structures, but figuring out their function at this scale will take more fieldwork and comparison. Identifying most earthworks as geoglyphs narrows the category but does not answer why they were built or how they were used over the 2,500-year span.

The radiocarbon dating does confirm that these structures were built well over a thousand years before Europeans arrived in the Amazon. That timeline matters because it clearly separates the work of Indigenous peoples from any changes made during the colonial period.

For researchers, the study raises questions about where the earthworks are clustered, how they were built, and how their locations relate to soil, water, and forest type. With over 20,000 structures documented, the dataset is large enough to support landscape-wide analysis that smaller surveys could not. Whether the earthworks represent a continuous building tradition across the full 2,500 years or separate bursts of construction with gaps in between is one question the data can begin to address.

The study also connects to present-day debates about Amazon conservation and Indigenous land rights. Finding this many precolonial earthworks reinforces the idea that large parts of what is now forest were actively managed or modified by human populations for long periods. That has implications for ecological models that treat the forest as untouched wilderness, and it feeds into legal and policy discussions about the long history of Indigenous occupation and stewardship.

Looking at what this means for the field, the scale of the dataset is the headline. Twenty thousand structures, most classified as geoglyphs, radiocarbon-dated to a 2,500-year range — these are the parameters within which the next phase of research will operate. The questions that follow are about comparison and function: how do these earthworks relate to sites elsewhere in the Amazon, what building techniques were used, and what population levels does this density of earthworks suggest?