Deep beneath the hillsides and suburbs of Brazil, there are tunnels so vast and regular that they look like abandoned subway lines. Their walls are smooth, their ceilings arched, and in some cases they stretch for hundreds of meters. Yet they are not mines, not sewers, and not the work of any known geological process. They are the fossilized burrows of vanished giants, carved into the rock long before humans ever set foot in South America.
These structures, known as paleoburrows, overturn the usual picture of caves as products of water and time. They are deliberate architecture, dug claw stroke by claw stroke by prehistoric megafauna that treated solid ground as if it were soft soil. To walk through one today is to step into a tunnel system that no human designed, but that still feels eerily engineered.
From “strange caves” to a new kind of fossil
The story of these tunnels begins with confusion. Early visitors assumed they were exploring ordinary caves, the kind that form when groundwater dissolves limestone or lava tubes drain away from cooling rock. Yet the shapes did not quite fit. The passages were too uniform, the floors too flat, and the ceilings too consistently curved to match the chaotic geometry of typical karst caverns or volcanic conduits. In some Brazilian hillsides, the openings appeared in clusters, as if something had systematically honeycombed the slope from within.
Only when researchers started looking closely at the walls did the true nature of these voids come into focus. Instead of the fractured textures of natural rock, they found parallel grooves and overlapping scratches, the unmistakable imprint of claws raking through sediment. In one widely cited case, investigators described how a seemingly natural cave ended in a chamber where the ceiling and sides were covered in dense claw marks, a pattern that ruled out both human tools and ordinary erosion and pointed instead to a powerful digging animal that had literally sculpted the space with its limbs, a conclusion reinforced by detailed observations of the claw marks on the ceiling.
What scientists mean by “paleoburrow”
Once the pattern became clear, researchers needed a name for these structures that captured both their age and their origin. They settled on a simple, literal term. A Paleoburrow is defined as an underground shelter excavated by extinct paleo-vertebrate megafauna that lived in prehistoric times, a category that includes the giant mammals that once dominated South America. Unlike ordinary trace fossils, such as footprints or feeding marks, these burrows are large enough to walk through and complex enough to count as full-scale habitats.
This definition matters because it separates the tunnels from both natural caves and human-made excavations. By treating them as a distinct class of fossil, scientists can systematically catalog their dimensions, map their distribution, and compare their internal features, from branching patterns to scratch orientations. The concept of the paleoburrow also helps fight misinformation, since it anchors the discussion in observable evidence of extinct animals rather than in speculation about lost civilizations or unexplained “mystery tunnels” that sometimes circulate in popular culture.
Brazil’s hidden network of megafauna tunnels
Once geologists and paleontologists knew what to look for, the scale of the phenomenon in Brazil became startling. What began as a handful of odd caves turned into a nationwide survey of underground architecture. According to reporting on megafauna paleoburrows in Brazil, geologist Heinrich Frank has identified more than 1,500 tunnels across Brazil, a figure that turns isolated curiosities into a continent-scale ecosystem of underground shelters.
These structures are not small animal holes. Some of the burrows span hundreds of meters, with a few reported to approach 2,000 feet in length, and their cross sections can be large enough for a person to stand upright or even for a small car to fit inside. The distribution of these tunnels across different Brazilian states suggests that multiple species of megafauna, living in varied environments, adopted burrowing as a core survival strategy. The sheer number, more than 1,500, implies that for a long stretch of prehistory, parts of Brazil were as riddled with animal-made tunnels as modern cities are with roads and utility lines.
The first clues in the Amazon
One of the turning points in understanding these structures came when researchers started to find them in places where no one expected large caves at all. In the western Amazon, a geologist named Amilcar Adamy of the CPRM, Brazil’s geological survey, encountered a strange opening in a hillside that did not match the usual patterns of river erosion or rock collapse. It was only later, when he had an opportunity to return and study it in detail, that the full significance of the find became clear.
Reporting on that work describes how The First such structure in the Amazon forced scientists to rethink what kinds of animals had lived there and how they had interacted with the landscape. The tunnel’s regular profile, its smoothed interior, and the absence of typical karst features pointed again to deliberate excavation. The fact that it appeared in the Amazon, far from the limestone belts that host many classic caves, strengthened the case that these were not geological accidents but the work of powerful diggers that had reshaped even the rainforest’s subsurface.
Surveying the giants’ handiwork
Turning scattered discoveries into a coherent picture required more than casual exploration. In 2010, Amilcar Adamy from the Brazilian Geological Survey began to scrutinize a particularly striking cave in Rondonia, western Brazil, using the tools of a modern Survey rather than the assumptions of traditional speleology. By carefully measuring the tunnel’s dimensions, mapping its branches, and documenting the orientation and depth of claw marks, he and his colleagues could test hypotheses about which animals had dug it and how they had used the space.
The work in Rondonia, Brazil, showed that these were not random scrapes enlarged by erosion but purpose-built corridors with consistent widths and heights, often sloping gently upward or downward in ways that would have made movement easier for a large quadruped. The Survey methods also helped distinguish overlapping generations of excavation, where younger animals might have expanded or reoccupied older burrows. By treating the tunnels as data-rich fossils rather than curiosities, Adamy and his peers turned them into a new window on South America’s lost megafauna.
Classifying “shelter for giants”
As the catalog of paleoburrows grew, scientists began to see patterns that justified formal classification. Not all tunnels were alike. Some were narrow and relatively short, while others were broad, branching, and monumental. To capture these differences, researchers proposed ichnotaxonomic categories, treating the burrows themselves as trace species. One such label, Megaichnus, was applied to a set of particularly large and distinctive structures.
Within this framework, the narrower and more abundant paleoburrows were given the classification Megaichnus minor. Most of these are filled with sediment today, which makes their original interior surfaces harder to study but also helps preserve them from collapse. The distinction between Megaichnus minor and larger forms hints at a community of different burrowing species, perhaps including both giant sloths and armored mammals like glyptodonts, each leaving its own architectural signature in the rock.
Why geology and humans are ruled out
For anyone used to thinking of caves as natural features or human projects, the claim that these tunnels are neither can sound bold. Yet the evidence that rules out geology and people is straightforward. Natural caves typically follow weaknesses in the rock, such as fractures or bedding planes, and their cross sections vary as water flow or lava dynamics change. In contrast, many paleoburrows maintain a remarkably constant diameter over long distances, with smooth, gently curved ceilings and floors that lack the scalloped textures of water erosion or the jagged edges of rockfall.
Human-made tunnels, from mining adits to modern infrastructure, leave their own signatures: tool marks with sharp, repetitive patterns, debris piles, and often some trace of structural support or cultural material. The South American tunnels lack these hallmarks. Instead, they display overlapping claw grooves, rounded corners where a large body repeatedly brushed past, and entrances that are too irregular and low for efficient human use. Detailed accounts of Scientists Unearth Giant Underground Tunnels, And They Weren built by humans or nature emphasize that the combination of scale, shape, and surface markings does not match any known geological process or historical construction technique, but fits well with the expectations for burrows dug by massive mammals.
The animals that could move mountains
Once geology and human engineering are off the table, the question becomes which animals were capable of such feats of excavation. South America’s fossil record offers several candidates. Giant ground sloths, some weighing several tons, had powerful forelimbs and massive claws that would have been well suited to digging. Armored herbivores like glyptodonts, relatives of modern armadillos but the size of small cars, also possessed strong limbs and protective shells that could have helped them push through compacted sediment while staying safe from predators.
Evidence from the tunnels themselves supports the idea that multiple species were involved. Variations in cross-sectional shape, from more circular to more elliptical, and differences in scratch patterns suggest that not all paleoburrows were made by the same kind of animal. Some reports describe claw marks that match the spacing and curvature expected from giant sloth forelimbs, while others hint at different digging styles. Accounts of Giant underground tunnels in South America, Not made by humans or natural forces, describe how some of these structures may even record subtle interactions with early humans, who encountered the abandoned burrows long after their original builders had vanished.
Living inside a megafauna subway
Understanding why these animals dug such extensive shelters is as important as identifying who did the digging. In a continent that experienced dramatic climatic swings, from wetter periods to cooler, drier phases, underground refuges would have offered stable temperatures and protection from both predators and environmental extremes. A multi-ton herbivore that could retreat into a tunnel during heat waves, storms, or wildfires would have had a significant survival advantage over one that had to endure conditions on the surface.
The internal layouts of some paleoburrows support this interpretation. Branching side chambers, widened sections that could serve as resting areas, and entrances oriented away from prevailing winds all point to deliberate design choices, even if they were the product of instinct rather than conscious planning. Reports on Scientists Unearth Giant Underground Tunnels note that the mysterious discovery of multiple interconnected passages suggests complex behavior, perhaps including communal use or repeated occupation over generations. In effect, these animals were not just digging holes, they were constructing long-term homes that reshaped the subsurface environment.
How the tunnels reshape our view of South American prehistory
The recognition of paleoburrows forces a revision of South America’s prehistoric landscape. Instead of picturing megafauna only as surface roamers, grazing across open plains or forest clearings, we now have to imagine them as engineers that modified the ground beneath their feet. Their tunnels would have influenced drainage patterns, soil mixing, and even plant communities, much as modern burrowing animals do on a smaller scale. Over thousands of years, networks of large burrows could have created microhabitats for smaller species, from reptiles to invertebrates, that took advantage of the stable conditions underground.
These structures also add a new dimension to the story of extinction. When the giant sloths, glyptodonts, and other burrowing megafauna disappeared, their tunnels remained as empty infrastructure, slowly filling with sediment or collapsing but still detectable in the rock record. Accounts of South America’s giant underground tunnels suggest that early humans may have encountered and even used some of these ready-made shelters, adding a layer of indirect interaction between our species and the vanished giants. The tunnels, in other words, are not just fossils of animals, they are fossils of behavior and of an entire subterranean dimension of ancient ecosystems.
The enduring mystery and what comes next
Despite the progress in identifying and classifying paleoburrows, many questions remain open. In some regions, the exact species responsible for particular tunnels are still debated, especially where skeletal remains are scarce or absent. The full geographic extent of these structures is also uncertain, since dense vegetation, urban development, and simple inaccessibility hide many potential sites. Even in Brazil, where more than 1,500 tunnels have been documented, researchers suspect that the known examples represent only a fraction of what lies underground.
What is clear is that these giant tunnels are real, extensive, and definitively not the work of humans. They are the physical legacy of animals that treated solid earth as a medium to be carved and shaped, leaving behind corridors large enough for us to walk through but alien in their origin. As more paleoburrows are mapped and analyzed, from the first finds in the Amazon to the carefully surveyed systems in Rondonia, the picture of South America’s past will continue to shift, revealing a continent once crisscrossed by the handiwork of creatures powerful enough to dig their own world beneath ours.
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