Humans did not build South America’s enormous mysterious tunnels. The structures are best explained as paleoburrows—ancient animal-made galleries excavated by extinct megafauna. Their claw marks, dimensions, and possible armor impressions point especially toward giant ground sloths and giant armadillo relatives, although the exact builder varies by site and remains uncertain in many cases.
The enormous tunnels found in parts of South America were probably not made by people at all. Researchers call them paleoburrows: underground galleries dug by extinct members of South America’s megafauna, including giant ground sloths and giant armadillo relatives.
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That answer is both more ordinary and more fascinating than a lost-civilization theory. The tunnels preserve evidence of animal behavior in the form of claw grooves, unusual wall textures, chambers, branching passages, and, at some sites, impressions that resemble the bony armor plates of extinct armored mammals.
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What is a paleoburrow?
A paleoburrow is a fossilized or partly preserved underground structure made by an extinct vertebrate. It is a type of trace fossil: evidence of what an animal did rather than a fossilized piece of the animal’s body.
That distinction matters. A fossil bone can identify an animal’s anatomy. A burrow can reveal behavior—where an animal moved, how it excavated, whether it returned to a shelter, and how its body fitted inside the passage.
South American paleoburrows can include:
- long, enclosed galleries;
- smooth or deliberately shaped walls;
- branching passages and chambers;
- ceiling and wall grooves consistent with repeated claw strokes; and
- surface impressions that may record contact with an excavator’s body or armor.
Some researchers have proposed the formal trace-fossil name Megaichnus for exceptionally large South American examples. The name describes the preserved structure and its characteristics; it does not mean that one particular animal has been identified as the builder of every tunnel assigned to that group.
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The case does not depend on the tunnels merely looking old or unusual. It rests on physical features inside them.
1. The walls bear repeated grooves
Some paleoburrow walls contain parallel or repeated grooves interpreted as claw-scrape marks. Their position and continuation along the walls and ceiling are difficult to explain as the product of a planned human excavation. They are much more consistent with a large animal pulling material away as it advanced through sediment or softer rock.
A single scratch would not settle the question. Geological cracks, erosion, tools, and later damage can all create marks. The significance comes from the pattern: the marks occur where a digging animal could have reached them and may repeat through the tunnel’s interior.
2. The tunnel’s shape can be compared with animal anatomy
Researchers can compare a passage’s height, width, curvature, and internal geometry with the likely body size and movement of extinct animals. A tunnel that is broad enough for a very large mammal, but shaped in a way that fits an animal excavating with powerful forelimbs or claws, provides behavioral evidence that a human-built explanation does not account for as well.
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At some sites, researchers have reported impressions resembling osteoderms—the bony plates embedded in the skin of armored xenarthrans. These impressions are not a signed label from a giant armadillo, and they must be interpreted carefully. But alongside claw marks and tunnel dimensions, they strengthen the case that at least some burrows were associated with extinct armored mammals.
Taken together, the evidence strongly supports biogenic excavation: the tunnels were made by animals. It does not automatically identify a single species, a single period, or a single purpose for every example.
Which animals could have dug them?
The main candidates belong to the xenarthrans, an unusual South American mammal group that includes modern sloths, armadillos, and anteaters. The extinct forms were often far larger and more heavily built than their living relatives.
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Giant ground sloths
The largest tunnels are often linked to extinct ground sloths, particularly mylodontid forms related to animals such as Glossotherium, Lestodon, and Valgipes. These animals had large bodies and robust limbs, making them plausible excavators of passages that are far too large for any living burrowing mammal in the region.
That does not mean the famous name Megatherium should be attached to every giant tunnel. Different ground-sloth groups had different limb and claw anatomy. Comparisons from some Minas Gerais examples have favored mylodontid candidates, while suggesting that certain megatheriids may not have matched the observed digging marks as well.
Giant armadillos and pampatheres
Smaller tunnels may fit giant armadillos, pampatheres, or other extinct cingulates—armored relatives of modern armadillos. Their claws and body proportions provide a plausible match for narrower galleries, while possible osteoderm impressions offer an additional clue.
Size is therefore a useful first filter:
| Burrow characteristic | More compatible candidates | What remains uncertain |
|---|---|---|
| Very large galleries and chambers | Large ground sloths, especially mylodontid forms | The exact sloth species and whether more than one animal used or modified the passage |
| Smaller, narrower passages | Giant armadillos, pampatheres, or related armored xenarthrans | Whether armor impressions belong to the original excavator or a later occupant |
| Grooves or scratches on walls and ceilings | An animal capable of excavating with claws or robust forelimbs | Marks can be altered by erosion, collapse, or later activity |
The safest scientific wording is “probable excavator” rather than “confirmed builder.” In many cases the animal itself is absent, and the structure may have been enlarged, damaged, or reused after it was first dug.
How old are the tunnels?
There is no single age for all South American paleoburrows. The broader fossil-burrow record includes Pliocene and Pleistocene contexts, and examples occur in more than one geological setting.
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Many Brazilian examples are discussed in connection with the late Pleistocene megafauna that disappeared around the end of the last Ice Age, roughly 10,000 years ago. That is useful context, but it should not be mistaken for a precise date assigned to every tunnel. Dating has to be done site by site, using the surrounding sediments, associated fossils, and other geological evidence.
The tunnels also occur in varied materials. Brazilian examples have been documented in sandstone and other sedimentary substrates, as well as iron-rich formations. A structure’s present appearance may reflect both the original excavation and later erosion or weathering.
Where are the paleoburrows?
Brazil is the principal modern research and media focus, but these are not the remains of one isolated tunnel system in one mysterious location. Paleoburrows have been documented in multiple Brazilian regions, including southern Brazil and Minas Gerais, and related fossil burrows are known elsewhere in South America, including Argentina.
The variety of locations is scientifically important. It suggests that burrowing by large extinct mammals was not a single local oddity. The surrounding rock, climate, available shelter, and animal communities varied from place to place, so researchers must interpret each site in its own geological and ecological context.
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Paleoburrows are also part of the educational and geological heritage recognized in Brazilian landscapes such as the Seridó UNESCO Global Geopark. That recognition does not turn every tunnel into a tourist cave or prove that every example is open to the public. It highlights the value of preserving these records of ancient life.
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Did humans ever use the tunnels?
Possibly—and this is where a catchy headline needs one important correction.
Evidence that people entered or occupied a paleoburrow at a later time would not show that people dug it. It would show reuse, not original construction. A cavity excavated by an extinct animal could later become a shelter, a temporary activity space, or a place visited by people.
Brazilian official and educational material documents paleoburrow visitation sites, and researchers consider later human use an important question. If tools, artwork, hearths, or other archaeological material were found in a tunnel, those discoveries would add a human chapter to the site’s history without changing the evidence for its animal origin.
So the accurate distinction is:
- Who made the original passage? Most likely extinct megafauna.
- Could people have entered it later? Yes, at least at some sites this is possible or documented as a subject of investigation.
- Does later occupation make it a human-made tunnel? No.
Why these animal-made tunnels matter
A paleoburrow is not simply an oversized hole. It is a three-dimensional record of an animal’s interaction with its environment.
Its dimensions may help estimate the excavator’s body size. The arrangement of chambers and branches can offer clues about movement, shelter, repeated use, or the animal’s digging strategy. The tunnel’s relationship to the surrounding sediment can reveal what kinds of ground were workable and how the landscape changed after excavation.
That makes paleoburrows a kind of paleoenvironmental archive. They preserve behavior that bones rarely can: the physical act of digging, the spaces an animal created, and the way other organisms may have interacted with those spaces afterward.
For animal lovers, they also provide a rare reminder that extinct mammals were not just large skeletons in a museum. They altered landscapes. Like modern badgers, rabbits, wombats, prairie dogs, and armadillos, they created shelter and reshaped the ground—only on a dramatically larger scale.
Readers who want a visual introduction to the animals behind the story may enjoy an illustrated giant sloth book. It is best treated as a general introduction to giant sloths and prehistoric life, not as a technical field guide to Brazilian paleoburrows.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why visitors should treat paleoburrows as fragile fossils
Their apparent toughness is deceptive. Paleoburrows can be damaged by uncontrolled visitation, flowing water, rock falls, construction, and the gradual loss of wall surfaces. A boot scrape or moved fragment may destroy evidence that cannot be reconstructed later.
That is why a paleoburrow should not be treated like an ordinary adventure cave. Do not enter an unapproved site, climb on its walls, remove sediment or rock, or publish the location of a vulnerable tunnel without following local conservation guidance. Official access rules, when a site is open, matter as much as the geological attraction itself.
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The conservation challenge is especially significant because the evidence is often subtle. A tunnel may remain recognizable even after the grooves, impressions, or sediment layers that make it scientifically valuable have been damaged.
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The strongest conclusion is not that scientists have identified one gigantic prehistoric tunneler. It is that the original passages were probably excavated by extinct South American megafauna, not by an ancient human civilization.
Large structures are more consistent with giant ground sloths, especially mylodontid relatives. Smaller structures are more consistent with giant armadillos, pampatheres, or other armored xenarthrans. Claw marks, tunnel geometry, and possible armor impressions make the animal-excavation explanation compelling, while the exact builder remains a site-specific question.
Humans may have reused some tunnels later, but reuse is not construction. The result is a more interesting story than the original mystery: these passages are trace fossils that preserve the engineering work of animals that vanished thousands of years ago.
Frequently Asked Questions
What are the mysterious South American tunnels called?
Researchers call them paleoburrows, or fossilized underground structures made by extinct vertebrates. They are trace fossils because the tunnel preserves an animal’s behavior rather than its body.
Which animals made the giant tunnels?
The evidence points to extinct megafauna, not a single confirmed species. The biggest passages are generally compatible with giant ground sloths, while smaller ones may have been made by giant armadillos, pampatheres, or other armored xenarthrans.
Could humans have used the tunnels?
Some tunnels may have been entered or reused by people later. That would document human reuse, not human excavation of the original passage.
Are all of the tunnels exactly 10,000 years old?
No. Paleoburrows occur in different geological contexts, including Pliocene and Pleistocene settings, so each site must be dated separately. Many Brazilian examples are associated with late-Pleistocene megafauna that disappeared around the end of the last Ice Age.
The Bottom Line
Bottom line: South America’s massive mysterious tunnels were probably excavated by extinct megafauna—especially giant ground sloths and giant armadillo relatives. Claw grooves, tunnel geometry, and possible armor impressions support animal excavation. Humans may have reused some passages, but they did not build the original tunnels, and no single species can be assigned to every site.
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