Intro: When a Cave Isn’t a Cave

At first glance, they look like ordinary caves—arched passageways that meander through hillsides, sometimes in networks with multiple entrances and side tunnels. But a closer look reveals something astonishing: the walls are etched with parallel grooves, the floors bear rhythmic furrows, and the cross-sections are oddly consistent—more like the inside of a gigantic, oval pipe than a natural cavern. These are not human mines, not water-carved caves, and not faults split open by earthquakes. They are paleoburrows: vast, ancient tunnels excavated by extinct megafauna during the last Ice Age.

The phrase “not built by humans or nature” is a catchy way to say “not by human engineering or by typical geological processes.” In reality, they are very much products of nature—of animals so large and powerful that they reshaped the ground beneath their feet. The story, covered in multiple outlets including Indian Defence Review, has sparked worldwide curiosity about who dug them, how, and why.

What Exactly Are Paleoburrows?

Paleoburrows are ancient animal-made tunnels preserved in rock-like soils (saprolite) and sediment. Most documented examples are found across southern and southeastern Brazil, with additional sites in Uruguay and other parts of South America. Many are big enough for humans to walk through comfortably. Some complexes extend for dozens of meters, occasionally branching into multi-entrance systems.

• Typical dimensions: Roughly 1–2 meters high, 1–2.5 meters wide; some passages are tighter or larger depending on the local substrate and species.
• Longevity: Dug during the late Pleistocene to early Holocene, then preserved as soils hardened and sediments filled in.
• Surface clues: Distinctive claw marks on walls and ceilings, often tens of centimeters long and etched in parallel sets, sometimes deep enough to hook a finger into.

Who Dug These Giants?

Evidence points to extinct giant ground sloths and giant armadillos, relatives of modern species but far larger and equipped with formidable claws. Candidates often mentioned by researchers include:

• Giant ground sloths (e.g., Glossotherium, Scelidotherium): Barrel-chested, powerful forelimbs, and long curved claws capable of gouging through consolidated soils.
• Large armadillo relatives (e.g., Pampatherium): Robust claws and digging adaptations; likely responsible for narrower, more uniform tunnels.

Matching tunnel morphology and claw-scratch patterns to specific species is ongoing work. In some burrows, the arc and spacing of scratch marks better fit a sloth’s curved claws; in others, the profile and rhythmic floor furrows suggest armadillos. It’s possible that different species dug different classes of burrows—or even modified one another’s work over time.

How Scientists Know These Aren’t Natural Caves

Several lines of evidence rule out typical geological origins:

• Tool marks vs. water marks: Walls display distinct claw grooves rather than solutional scallops or fracture patterns common to karst caves.
• Consistent, oval cross-sections: Many burrows maintain a remarkably even arched profile, unlike the irregular voids left by erosion or collapse.
• Sediment fill and layering: Interiors often contain reworked soils and collapsed material consistent with long-term occupation and later infill—not mineral deposits formed by groundwater.
• Absence of human artifacts: No pick marks, structural supports, or cultural materials typical of mines or ancient construction.

Researchers have documented hundreds of these features, mapping them with photogrammetry and 3D laser scanning to analyze their geometry, internal textures, and wear patterns. The resulting models bolster the interpretation that they are the product of repeated, deliberate excavation by large burrowing mammals.

Why Dig? Hypotheses About Function

Animal-built architecture is common in modern ecosystems; think of prairie dog towns, wombat burrows, or aardvark dens. Scale that up to Ice Age proportions and you get paleoburrows—potentially multifunctional structures providing:

• Shelter and thermoregulation: Underground refuges buffer temperature and humidity, critical during climatic swings.
• Protection from predators: Large animals, especially juveniles, gain security underground.
• Rearing sites: Larger chambers and branching passages may have served as nesting or communal spaces.
• Seasonal refugia: Animals could retreat during droughts, wildfires, or harsh winters.

The sheer labor involved implies repeated or long-term use across generations. Some complexes likely grew incrementally as occupants enlarged, repaired, or re-excavated passages.

Where They’re Found

The best-known paleoburrows occur in southern Brazil (Rio Grande do Sul, Santa Catarina), with others in states like Minas Gerais and São Paulo, and across neighboring regions in South America. Local terms like paleotoca reflect how embedded these features have become in regional geology and paleontology.

Many occur in weathered volcanic or sedimentary terrains where soil profiles are thick and cohesive enough to hold a tunnel shape after excavation, yet soft enough for an animal—albeit a massive one—to dig.

Claw Marks as a Rosetta Stone

The most compelling “signature” inside these tunnels is the claw mark. Researchers analyze:

• Spacing and parallelism: Sets of grooves align like the fingers of a digging hand.
• Curvature and depth: Match the arc of giant sloth claws or the robust, chisel-like digits of armadillos.
• Orientation: Ceiling and wall marks reflect the animal’s stance and digging motion; floor furrows show body drag or repeated traffic.

Micromorphology can even distinguish fresh cuts from later abrasion, helping reconstruct burrow phases and maintenance cycles.

Dating the Underground

Directly dating the act of digging is difficult. Instead, scientists date sediments that later filled the tunnels, overlying deposits, or associated fossils. Results generally place activity in the late Pleistocene to early Holocene, a window when giant ground sloths and large armadillo relatives still roamed South America. Continued work aims to refine these chronologies with improved sampling and multiple dating techniques.

Ecosystem Engineers of the Ice Age

If megafauna dug vast networks of refuges, they didn’t just shelter themselves—they reshaped soils, hydrology, and habitats, much like today’s beavers or elephants. Long after the animals vanished, the burrows persisted, subtly guiding water flow, providing dens for smaller creatures, and preserving snapshots of ancient climates in their sediment fills.

Recognizing paleoburrows reframes our understanding of past ecosystems: underground architecture may have been as important as above-ground vegetation in structuring communities.

Open Questions

• Species attribution: Can we reliably match specific burrow types to particular taxa using claw metrics and biomechanics?
• Social behavior: Were some complexes communal, or do they reflect repeated reuse over centuries?
• Geographic extent: How widespread are paleoburrows beyond known hotspots? Are we undercounting them beneath farmland and forests?
• Paleoenvironment: What do fills reveal about climate pulses, vegetation shifts, and faunal turnover?

Conservation and Responsible Access

Paleoburrows are fragile. Their walls can crumble under vibration or careless excavation, and untrained visitors may damage diagnostic claw marks. Researchers recommend:

• Surveying with noninvasive methods (photogrammetry, handheld LiDAR) where possible.
• Avoiding disturbance of sediment fills that preserve stratigraphic records.
• Coordinating with landowners and local authorities to protect sites.

Behind the Headlines

Sensational titles like “They weren’t built by humans or nature” are designed to grab attention. The reality is no less wondrous: these tunnels are nature’s work—by animals so large and industrious that they created enduring subterranean landscapes. Coverage in outlets such as Indian Defence Review helped propel the story to broader audiences, but the scientific intrigue stands on its own: we’re uncovering a hidden chapter of Earth’s engineering, authored by claws and muscle during a colder, wilder time.