Scattered across the vast grassy floodplains of the great Orinoco River of South America, in an area roughly the size of Ireland, are thousands of strange earthen mounds known among locals as “surales”. Regularly spaced and densely packed, these mounds can reach up to 6 feet tall and 16 feet wide. The mounds were first described in the 1940s, but no scientific studies were ever conducted because they were thought to be man-made.

Now researchers have found that surales’ true architects are gigantic earthworms that reach 3 feet in length. These earthworms, belonging to the genus Andiorrhinus, are the largest earthworms found in the area. They feed on organic materials found in the soil and poop a convoluted mass of soil, mud, and sand known as “casts” that pile up to form these mounds.

An aerial view of the tropical wetlands showing hundreds of mounds. Photo credit: José Iriarte