Just when you think nature has maxed out on weird phenomena, here come the poop-piling worms of South America. Researchers say enormous earthworms are likely responsible for the huge mounds covering tens of thousands of square miles in the wetlands of Columbia and Venezuela. The scientists published their report in the journal PLOS One.

Those mounds are called surales. These broad green lumps on the landscape can reach more than 16 feet across. There are scores of them, patterning the wetlands like huge green polka dots. There are several types of patterned landscapes, the authors note in their paper, but surales “… must be seen to be believed.” Yet somehow these mysterious mounds have lain largely overlooked and uninvestigated.

Aerial view of surales landscape. Image credit: Delphine Renard

There are two primary theories about the mounds’ formation. One is that they were built by the movements of small animals, insects, or worms; and the other is that they’re the natural product of erosion around circles of plant roots. “Why the plants are regularly spaced has not been addressed in these cases,” write the authors of the current paper. “In addition, we found no authoritative source that demonstrates this phenomenon or even offers an explanation of how it could work.” In other words, they weren't buying it.

Residents of the Orinoco plains are pretty sure worms were responsible, and their names for the mounds (tatuco, zuro, zural, and sural) all refer to piles of worm poop. This is less outrageous than it might sound. Earthworms are plentiful on the plains, and some of them are gargantuan. The researchers decided to figure out if earthworms were making the mounds, and, if so, which earthworms.

The scientists went at the question from all angles, taking samples of plants, soil, stone, and worms, and scanning the landscape using images from aerial drones and Google Earth.

The combination of micro- and macro-level analysis revealed some answers—and some surprises. "We were really impressed not only by the regularity in the size and spacing of mounds in surales landscapes, but also by their spatial extent,” co-author Anne Zangerlé of Technische Universität Braunschweig said in a press statement. “We showed that they occur throughout much of the Orinoco Llanos, in both Colombia and Venezuela, but they have hardly been noticed by ecologists. We were surprised to find that the main driver of these earth-mound landscapes appears to be a single very large earthworm species."

When Zangerlé says “very large,” she’s not kidding. Juvenile Andiorrhinus sp. earthworms can reach more than 3 feet long. There are also a lot of them: This single species makes up for 92.9 percent of local earthworm biomass. They’re kind of in charge. 

To make the mounds, these big, juicy worms (or “soil engineers,” as the researchers call them) suck in muddy wetlands water, process the soil, and then poop it out in dry, hardened towers called castings. Those towers are not just for show; the worms climb up them to get out of the water and breathe. The castings pile up, and they combine to form the once-mysterious mounds.