The Mysterious Mega-Tunnels of Brazil: Ancient Passages That Weren’t Made by Humans

If you hike the red-earth hills of southern Brazil, you might stumble into an opening in the slope: an arched hallway tall enough to walk through, its walls scored with deep parallel grooves. Locals long assumed these were hideouts carved by Indigenous people or smugglers’ routes. But when geologists crawled inside with measuring tapes and headlamps, they realized the truth was far stranger. These are paleoburrows—vast underground tunnels dug not by humans, but by extinct mega-mammals that roamed South America during the last ice age. 

What exactly is a paleoburrow?

A paleoburrow is a tunnel or chamber excavated by prehistoric animals. In Brazil, they form a newly appreciated category of “trace fossils”—physical records of ancient behavior rather than bones or teeth. Many of these passages are smooth, elliptical tubes that branch, climb, and descend through soft rock and soil. Their most distinctive feature is the tool-marks left by the engineers themselves: claw rakes that run in sets across ceilings and walls like the combed grooves of a giant hand. Those claw marks are why scientists concluded the tunnels weren’t made by water, volcanism, or people—they are literally scratch-signed by their makers. 

Where are they found?

Although paleoburrows exist in several parts of South America, the greatest concentration is in Brazil, especially in the southern states of Rio Grande do Sul and Santa Catarina, with others in Minas Gerais and even in Rondônia, at the southern edge of the Amazon. In southern Brazil alone, researchers have documented well over 1,500 tunnels, with new ones reported as highway cuts and construction projects expose fresh hillsides. 

How big are these tunnels?

Some paleoburrows are snug tubes barely wider than a person’s shoulders. But others are colossal. One network in Rondônia, first surveyed by geologist Amílcar Adamy of Brazil’s Geological Survey, totals roughly 600 meters (about 2,000 feet) of branching passage. The main galleries in such systems can exceed two meters high—large enough for a human to stroll through upright. Engineers estimate that thousands of tons of sediment were removed to create them, likely over generations. In Minas Gerais, chambers reach five to ten meters wide and up to four meters high, sometimes with multiple “resting hollows” carved into the floor. 

Who did the digging?

Two sets of suspects dominate the lineup, and both are now extinct:

• Giant ground sloths—imagine an animal as heavy as a small elephant, with enormous forelimbs ending in curved claws. Several South American genera, including Lestodon and Glossotherium, had anatomy suited to serious earth-moving. Many of the largest burrows, up to ~5 feet (1.5 meters) across, are attributed to them.

• Giant armadillos and their kin—not the small insect-eaters alive today, but hefty armored mammals like Pampatherium or Holmesina. These would have produced smaller-diameter tunnels, though some researchers think they could be responsible for sizable systems too.

Claw-mark spacing and shape help discriminate between potential makers, and the overall scale of a tunnel hints at the body size of the digger. But because bones are rarely found inside, scientists rely on these forensic clues rather than direct skeletal evidence.

How old are they?

Dating a hole is tricky. The animals themselves vanished from Brazil roughly 10,000–12,000 years ago, during the end-Pleistocene extinction that swept away much of the world’s megafauna. That gives a minimum age for the last active use of many burrows. Sediments that later washed in can sometimes be dated, and weathering of the walls also offers clues. The consensus: a large fraction of Brazil’s paleoburrows were dug during the late Ice Age, with some possibly older. 

Why build such enormous tunnels?

Here the story shifts from evidence to hypotheses. Large mammals often dig to escape heat, cold, parasites, and predators, to den and rear young, or to reach roots and tubers. In subtropical and tropical Brazil, a stable underground microclimate would have been attractive during climatic swings. The location of many burrows on slopes with good drainage suggests careful site selection. In a few valleys, multiple nearby tunnels and multi-hollow chambers hint at group living or repeated occupation—a kind of sloth suburbia used across generations. But until we find more direct traces—like coprolites (fossil dung) or preserved nest material—behavioral reconstructions remain educated guesswork. 

How do we know they aren’t natural caves?

Veteran cave geologists point to several telltale contrasts. Natural solution caves in limestone form by dissolving rock and tend to create sinuous conduits with scallops and speleothems (stalactites, stalagmites). Brazil’s paleoburrows, by contrast, often occur in sandstone, saprolite, or weathered volcanic rock that is not prone to forming classic karst caverns. Their cross-sections are consistently elliptical, the walls are tool-marked rather than chemically smoothed, and the passages branch and terminate in ways consistent with digging behavior, not groundwater flow. As one researcher put it, there’s no known geological process that produces long, branching tunnels with rhythmic claw striations across every surface. 

A brief history of discovery

Local farmers and road crews have known about these holes for generations, but the scientific story accelerated this century. In 2008, geologist Heinrich Frank of the Federal University of Rio Grande do Sul stopped at a fresh highway cut near Porto Alegre, crawled into an inviting arch, and realized he wasn’t in a normal cave. Over the next few years, he and colleagues documented hundreds, then thousands, of similar features. Independently, in 2010–2015, Amílcar Adamy followed rumors of a “strange cave” in Rondônia and surveyed what remains the largest paleoburrow network yet mapped in the Amazon. Their efforts seeded Brazil’s Paleoburrows Project, a collaboration raising awareness and pushing for protection of these fragile traces. 

What do the claw marks say?

Inside well-preserved tunnels, the walls look like they’ve been raked by garden tools the size of swords. The marks are typically parallel grooves a few centimeters apart—exactly what you’d expect from multi-clawed forelimbs pulling sediment backward. On ceilings, the streaks indicate animals rearing or scraping above shoulder height. In places, the scratches cross, suggesting multiple episodes of digging, resurfacing, or burrow enlargement across seasons or generations. These are the signatures that convince even skeptical geologists they’re standing inside an engineered space, not a freak of erosion. 

Why Brazil?

Giant ground sloths and large armadillos once lived across the Americas, yet most known paleoburrows cluster in southern and southeastern Brazil. Part of this is research bias—Brazilian scientists started looking and therefore found more—but geology and climate help, too. The right combination of soft, diggable bedrock and long periods of gentle erosion preserved these structures near today’s ground surface, where road cuts and rivers reveal them. As awareness spreads, more are being recognized elsewhere in South America, hinting that Brazil may be the heart of a broader, under-mapped phenomenon.

Threats and preservation

Because paleoburrows often lie just inside hillsides, they’re vulnerable to construction, quarrying, and casual vandalism. Some famous tunnels now carry the scars of modern visitors—spray paint over prehistoric claw marks, collapsed sections from careless digging. Researchers are advocating to catalog and protect representative sites as geologic and paleontological heritage. A 2024 academic review even frames paleoburrows as a class of geoheritage worth conserving alongside fossils and volcanic landscapes. Time is pressing: every rainy season washes in sediment; every unregulated visit risks damage to walls that have survived for millennia. 

What paleoburrows reveal about lost worlds

Bones tell us what animals looked like; burrows tell us what they did. Brazil’s tunnels imply that some Ice Age mammals were not just powerful diggers but landscape engineers, reshaping hillsides and soils. They suggest site fidelity (returning to the same dens), possible social behavior in clustered valleys, and an underground dimension to Ice Age ecosystems that rarely fossilizes. They also remind us that extinction erases behaviors as well as bodies. To walk a Brazilian paleoburrow today is to slip—briefly—into the habitual pathways of giants that vanished when climates shifted and humans spread across the continent. 

Seeing one for yourself

If your travels take you to Rio Grande do Sul or Santa Catarina, local guides and researchers occasionally lead visits to accessible paleoburrows (some on private land). Approach them like any archaeological site: look, don’t touch, and never add your own marks to the walls. Every groove is a sentence in a 10,000-year-old story. And in Brazil’s hills, those stories run for hundreds of meters—meticulously scraped by claws, not carved by human hands. 

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Further reading: For an engaging overview with measurements and interviews from field scientists, see Discover Magazine’s feature on Brazil’s paleoburrows. Public broadcasters have also covered expeditions into the tunnels and the campaign to protect them. For more technical work on distribution, morphology, and heritage value, consult recent academic papers on paleoburrows in southern Brazil.