Huge tunnels discovered that were not made by humans or geology

Deep under the hills of southern Brazil and northern Argentina, scientists found huge tunnels that appear to be cut into solid rock. They say no human or geological process created them.

They do not follow river channels, they show no signs of mining, and they look nothing like normal caves.

Many of the passages are longer than 600 yards (550 meters) and tall enough for an adult to walk through without bending. 

The leading idea is that giant, extinct ground sloths dug these colossal shelters, turning parts of South America into a maze of underground homes.

Unearthing the ancient tunnels

Over the past decade, a detailed study mapped more than 1,500 giant burrows across southern and southeastern Brazil. 

These tunnels can reach several hundred feet in length. They branch into side passages, and display long, parallel claw marks etched into their walls.

The work was led by Heinrich Frank, a geologist at the Federal University of Rio Grande do Sul in Brazil. His research centers on paleoburrows – fossil tunnels carved by large, extinct animals that once reshaped the landscapes of southern South America.

Many passages appear in consolidated sands, sandstone, or weathered volcanic rock. These materials are hard for machines to excavate, and harder for humans with simple tools. 

Collapsed ceilings and overlapping tunnels show that some routes were widened and reused, a pattern outlined in a chapter on Cenozoic tunnels.

Underground network of tunnels

Geological processes such as landslides, joints, and natural caves rarely create long, nearly circular tunnels that slope up and down or branch like these. 

Frank notes that the tunnel walls are packed with claw marks, sometimes in three parallel grooves, right where a digging limb would bite into rock.

Similar tunnels crop out along road cuts in Argentina, where they intersect and crisscross in dense clusters on some hillsides. 

Taken together, the layout looks less like an accident of erosion and more like a network of shelters dug and maintained over long periods.

Tunnel clues point to giant sloths

To identify the tunnel makers, scientists matched burrow size and claw patterns with fossil skeletons from the same regions. 

The biggest tunnels are at least 6 feet (1.8 meters) across and roughly as tall. This narrows the candidates to giant ground sloths and armadillos.

The claw traces are broad and shallow, matching the long, curved claws of sloths more closely than the shorter claws of armored diggers. 

These burrows are a textbook example of megafauna reshaping the ground as they moved and rested.

One top candidate is Megatherium, the best known South American giant ground sloth from the late Ice Age. Fossils suggest Megatherium weighed up to four tons and stood 12 feet (3.7 meters) tall. This is similar in size to an elephant, as noted in a museum piece.

It had a long tail for balance and massive forelimbs tipped with curved claws. A sloth built like this could rear up, brace, and dig steadily into sediment or softer rock over many generations.

Humans, giant sloths, and tunnels

These tunnels date to the Pleistocene, an ice age period that ended about 11,700 years ago. During this time, humans and giant sloths shared the Americas. 

In New Mexico’s White Sands, a footprint paper describes trackways where barefoot humans stepped into sloth prints and then followed them for distances.

“Human interactions with sloths are probably better interpreted in the context of stalking and/or hunting,” wrote David Bustos, a park scientist.

The trackways show that sloths sometimes turned to face their pursuers, whose footprints reveal where they clustered together. This left circular patterns where the sloths reared and lashed out with their claws. 

“Their strong arms and sharp claws gave them a lethal reach and clear advantage in close-quarter encounters,” wrote Matthew Bennett, a geologist.

Humans in North America were bold enough to stalk such dangerous animals across open lakebeds. It is thus highly possible that people farther south also hunted them. 

In that context, long, underground sloth burrows would have helped sloths avoid hunters, big cats, and sudden swings in temperature on exposed slopes.

Why paleoburrows matter today

Each paleoburrow preserves details that bones alone cannot. These include the shape, size and curve of the tunnel, and the texture of its scratch-covered walls and floors. 

They are a kind of trace fossil that preserves evidence of ancient activity, rather than fossilized bodies. Other trace fossils include footprints, feeding marks, nests and coprolites.

Taken with surface fossils, these underground records help scientists map where different sloth species lived and how they divided up habitats across the Americas. 

A review of Pleistocene sloths shows that they lived from grasslands to forest edges. Paleoburrows add an important behavioral context to these fossil ranges.

The tunnels also feed into efforts to understand how the loss of large animals changed soils and vegetation after the Ice Age. 

Lessons from giant sloth tunnels

Studies of extinct megafauna in many regions find that their disappearance reshaped ecosystems and nutrient flows.

As more paleoburrows are surveyed and mapped, they will link what we know from fossils and footprints into a picture of Ice Age life. 

The giant tunnels under Brazil and Argentina become more than curiosities; they stand as traces of how ancient sloths, people, and landscapes shaped one another during Ice Age times.

Image credit: R/caving, Giant Sloth Burrow

The study is published in Science Advances.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–