An aerial view, taken by a drone, shows a surale site in Los Llanos, the seasonally flooded savannah that spans from Colombia into Venezuela.

Large, mysterious mounds of soil found in the tropical grasslands of Los Llanos in South America finally have a scientific explanation: giant worms.

The mounds, found near the Orinoco River in Columbia and Venezuela, are called surales. Now, researchers have discovered that half of the mass of these dense soil piles is composed of earthworm excrement. The mounds form when worms — many reaching more than 3 feet (1 meter) in length — digest the dirt in the shallowly flooded grasslands of Los Llanos, researchers report today (May 11) in the journal PLOS ONE.

As they feed on the organic material in the soil, the worms excrete "casts," which are essentially worm poop. The casts pile up to form mounds 1.6 feet to 16.4 feet (0.5 to 5 m) in diameter. The surales can grow as high as 6.5 feet (2 m) tall. [14 Strangest Sites on Google Earth]

"This exciting discovery allows us to map and understand how these massive landscapes were formed," study researcher José Iriarte, an archaeologist from the University of Exeter, in the United Kingdom, said in a statement. "The fact we know they were created by earthworms across the seasonally flooded savannahs of South America will certainly change how we think about human versus naturally built landscapes in the region."

A growing surale in the seasonally flooded grasslands of Venezuela and Colombia. Surales are formed when worms return to the same spot again and again to feed on soil and deposit their excrement, or casts, in a muddy pile, researchers report in the journal PLOS ONE. (Image credit: José Iriarte)

Surales landscapes are striking. From air, they look bumpy and lumpy. On the ground, this view coalesces into a marshy grassland consisting of large, vegetated mounds separated by swampy ditches. Though people have generally attributed the soil patterns to worms, alternative explanations included termite activity or erosion, Iriarte and his colleagues wrote. No one had ever ruled these explanations out. In fact, no one had ever scientifically described the surales landscape and surales formation at all.

In their new study, the researchers used aerial and satellite photography as well as field studies of soil properties and soil organisms to examine the regular spatial pattern of the surales. The analyses found no evidence of termite activity, but plenty of busy earthworms — nine species, to be exact. A single species of giant Andiorrhinus worm was most prominent, making up almost 93 percent of worm biomass (meaning the total mass of worms at the field sites). Worms were much more prevalent in surales mounds than they were in the ditches surrounding them, and sometimes they couldn't be found in the ditches at all, though their burrows were present, the researchers said.

José Iriarte of the University of Exeter measures surales in South America with a colleague. These muddy features, made largely of earthworm poop, can grow to 6.5 feet (2 meters) tall and 16.4 feet (5 meters) in diameter. (Image credit: José Iriarte)

The mounds were about half earthworm casts by volume, the researchers found, and that percentage was higher than it was in the between-mound ditches, where the soil comprised between 0 percent and about 35 percent earthworm casts. Andiorrhinus, a true giant of a worm that can grow more than 3 feet (1 meter) long, appeared to be the main mound builder, Iriarte and his colleagues reported. The worms forage in shallowly flooded soils and then crawl to higher ground to breathe and excrete. Their casts form towers, which the worms return to again and again, perhaps over many generations, the researchers wrote. As the towers grow into mounds, the worms excavate the basins around them in search of more food, creating a self-perpetuating loop of lower and higher ground.

More questions remain, the researchers wrote, such as what happens when mounds eventually erode and collapse. Worm landscape formations are also known to be present in South Africa, Uganda and New Guinea, Iriarte and his colleagues wrote.

"Comparative study of these landscapes and the worms that make them would be most enlightening," they wrote.

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