Several colossal burrows have been discovered in South America. Yet as these tunnels appear to be so huge and neatly constructed, you'd be forgiven for thinking that they were dug up by humans.
Surprisingly, these tunnels appear to be far more ancient than they look. In fact they're estimated to be at least 8,000 to 10,000 years old. At first, no know geologic process could explain how these tunnels were made. However, due to the massive claw marks that line the walls and the ceilings, it is now believed that an extinct species of giant ground sloth is behind some of these tunnels (known as palaeoburrows).
Researchers have known about these tunnels since the 1930s. But at the time they were considered to be some kind of archaeological structure. In fact, they were thought to be remains of caves carved out by our ancient ancestors.
In 2010, geologist Amilcar Adamy from the Brazilian Geological Survey decided to investigate rumors of a peculiar cave in the state of Rondonia. Adamy initially intended to investigate the tunnels, determined to attribute them to some kind of geological process. However, upon seeing the tunnels with his own eyes, he couldn't think of any natural process that would create such a deliberate-looking structure. In fact, according to Adamy, the tunnels did not even appear to be remotely natural.
A professor at the Federal University of Rio Grande do Sul, Heinrich Frank, also discovered a strange cave, in the town of Novo Hamburgo. But this wasn't his only discovery. Once he knew what to look for, Heinrich found hundreds of them scattered across the Brazilian landscape.
Now there are more than 1,500 known palaeoburrows in southern and southeastern Brazil alone. There also appear to be two different types: the smaller tunnels measure 1.5 meters in diameter and the bigger ones can stretch up to 2 meters in height and 4 meters in width.
When Heinrich first looked up he got his first big clue about what could be behind their construction - distinctive grooves in the weathered granite, basalt, and sandstone surfaces - which he identified as the claw marks of a massive, ancient creature.
In a 2016 paper Heinrich explained that the caves had "long, shallow grooves parallel to each other, grouped and apparently produced by two or three claws. The groves are most smooth, but some irregular ones may have been produced by broken claws."
This discovery answered one of the long-standing questions in paleontology (the study of fossils), regarding the ancient magafauna that roamed the planet during the Pleistocene epoch - the geological epoch which lasted from about 2.5 million years to 11,700 years ago: where were all the burrows? It is estimated that about half of the mammalian species on Earth right now are classified as semi-fossoral. This means that they spend some time inside burrows, but exit to feed.
Considering that all the world's species have evolved from the more ancient versions of themselves it is believed that similar proportions of fossorial and semi-fossorial species would have existed around the time of the Pleistocene megafauna.
An intriguing revelation, though, is that despite the abundance of fossilised remains proving the existence of these creatures, for centuries, researchers could not identify any evidence of burrows. This could be due to a combination of, burrows collapsing over thousands of years, and researchers not being aware of what to look for.
Today, based on the size of the structures and the claw marks left in the walls, researchers are confident that they've discovered megafauna burrows, and believe that giant ground sloths and giant armadillos are likely the owners of these tunnels. In fact, Heinrich says that "there is no geological process in the world that produces long tunnels with a circular or elliptical cross-section, which branch and rise and fall, and with claw marks on the walls."
Heinrich also says that he's seen dozens of caves that have inorganic origins, and in these cases, it is clear that digging animals had no role in their creation."
The diagram below illustrates how the various tunnel diameters match up to the known species of ancient armadillos and sloths.
Yet while these creatures stood tall at 4.6 meters and weighed around 2,590kg, a single ground sloth would have likely spent much of its lifespan constructing tunnels. So why bother? Heinrich and his team aren't sure if the tunnels were used to escape the climate, predators, or humidity - however, all of these explanations seem unlikely, as a much smaller burrow would have served well. Perhaps several armadillos or sloths inherited the burrows over generations and kept adding to the structure. However further research and observation is required to determine the exact reason.
As yet, so many questions are still left to answer. But for now, let us appreciate this new finding. Meanwhile, if you're curious to discover more about giant sloths, check out this video below: