Across northeastern Brazil, an extraordinary landscape hides a secret beneath its surface. Around 200 million earthen mounds stretch across roughly 230,000 square kilometres, an area about the size of Great Britain. Some of these structures are almost 4,000 years old, yet they are not termite nests in the usual sense. Instead, they are enormous piles of soil excavated as Syntermes dirus termites carved out a vast network of interconnected underground tunnels. Over generations, the insects repeatedly carried earth to the surface, creating millions of evenly spaced cones. What looks like a field of ancient nests is actually the product of a hidden subterranean system.
200 million termite mounds contain over 10 cubic kilometres of soil
According to the study published in Cell Biology, titled тАШA vast 4,000-year-old spatial pattern of termite moundsтАЩ, the age of the landscape is another part of the mystery. Researchers collected soil from the centres of 11 mounds and used optically stimulated luminescence dating to determine when the material had been deposited. The resulting ages ranged from about 690 to 3,820 years ago, showing that some of the mounds have persisted for almost 4,000 years. The researchers noted that these ages are comparable with some of the oldest termite mounds known from Africa. The ancient dates also point to a process that continued over a very long period rather than a landscape created in a single episode. The researchers estimated that each mound contains about 50 cubic metres of soil. Across roughly 200 million mounds, that amounts to more than 10 cubic kilometres of excavated earth, which they compared with the volume of around 4,000 Great Pyramids of Giza. The study described the result as the greatest known example of ecosystem engineering by a single insect species.
BrazilтАЩs termite mounds have no chambers inside
The researchers inspected hundreds of structures exposed by road construction and carried out their own excavations. Rather than finding the chambers, passages and other elaborate features normally associated with termite nests, they found that each mound was essentially an amorphous mass of soil without an internal structure. Newly forming mounds contained a large central tunnel underground and connected to an extensive network of passages and narrow horizontal galleries. Some of these galleries contained harvested pieces of dead leaves or termite brood. The researchers found no royal chamber in or beneath the mounds' extensive searches, while the tunnels were sealed rather than left open to the outside, indicating that they were not simply ventilation structures.
Why termites built the vast underground tunnel network
The researchers proposed that the enormous tunnel network developed because the termites needed reliable access to food in a difficult environment. The caatinga forests of northeastern Brazil are seasonally dry, and leaf-fall occurs episodically. At night, groups of roughly 10 to 50 workers and soldiers emerge from temporary openings between the mounds to collect available food. Those small tubes are then sealed after use. According to the researchers, the permanent underground network allowed the termites to move safely through the landscape while reducing the effort needed to dispose of excavated soil. Rather than constructing each mound as a living chamber, the insects appear to have deposited the waste material generated by their underground construction. Over thousands of years, this repeated process produced the strikingly regular pattern visible across the Brazilian landscape.
BrazilтАЩs 200 million termite mounds are evenly spaced
The researchers measured the distances between neighbouring mounds and found an average separation of about 20 metres. Their analysis showed the mounds were dispersed, meaning they were more evenly distributed than would be expected from a random arrangement. With an estimated density of about 1,800 mounds per square kilometre, the researchers estimated approximately 200 million mounds. The study considered whether competition between termite colonies could explain the spacing, but experiments did not support that explanation. S. dirus workers and soldiers did not show aggression when encountered around the mounds, although termites placed in contact with individuals from 50 kilometres away became aggressive immediately. Instead, the researchers suggested that the spacing emerged through self-organisation as the insects used their interconnected tunnels to reach food and dispose of waste efficiently.
