Rising above the waterlogged grass plains of northern Columbia and southern Venezuela are strange mounds that stud the landscape and have baffled scientists for decades.

Now researchers have found the regularly spaced, densely packed hummocks of soil are made by giant earthworms as they burrow through the muddy Llanos wetlands around the Orinoco river.

The spectacular mounds, also known as surales, can reach up to 16ft (five metres) in diameter and rise up to 6ft (two metres) high at regular intervals across the landscape.

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Studding the wetlands around the Orinoco river is a bizarre mosaic of mounds (pictured) that can be seen from space. Their formation has puzzled scientists for decades but now a new study has revealed they are created by the soil expelled by burrowing giant earthworms as they dig through the waterlogged soil

They cover such a vast area that they can be clearly seen from space.

Researchers have previously assumed this striking landscape feature was formed by erosion caused by flooding of the Orinoco River.

WHAT ARE THE SURALES? First described in the 1940s by scientists, these strange mounds were spotted forming a moisiac across the flood plains around the Orinoco and its tributaries in northern Columbia and southern Venezuela. The mounds are known among locals as surales and can grow to be up to 6ft-tall tall and 16ft-wide. In some cases they form rounded domes while others appear to have joined together for create a labyrinth-like structure. Each mound is formed of a densely packed soil, often entangled with plant roots and other material. Aerial surveys have now revealed they cover a vast expanse of almost 29,000 square miles. The new research has revealed they are formed by the excreted material from a previously undescribed species of burrowing giant earthworms. Advertisement

They were first identified in the 1940s when descriptions of the strange earth mounds on the floodplains around the Orinoco and its tributaries were sent back to Europe.

But a new study published in the journal Public Library of Sciences One, has found the mounds are actually the result of intensive activity of one species of large earthworm.

The previously unknown worms, which belong to the South American genus Adiorrhinus and can grow up to 5ft (1.5 metres) long, expel the soil from the guts while burrowing through the soggy ground.

Professor José Iriarte, an archaeologist at the University of Exeter who was one of those involved in the study said: 'This exciting discovery allows us to map and understand how these massive landscapes were formed.

'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 verses naturally-built landscapes in the region.'

The researchers used aerial photographs taken by drones and satellite images to help them study the landscape along with samples taken from the surales.

Some of the mounds form rounded shapes while others join together to create vast labyrinths across the landscape, covering a vast area of nearly 29,000 square miles.

The surales form as soil is expelled by burrowing earthworms. Their casts eventually tower above the waterlogged landscape and build up into giant mounds (a developing surales is pictured)

The mounds are found across an expanse of wetland known as the Llanos that surround the Orinoco river and its tributaries in northern Colombia and Venezuela (shown on the map above)

The researchers analysed the chemical and physical make-up of the soil.

Their study found that earthworm casts account for around one half of the total soil mass of surales.

They form when the earthworms feed in the flooded soil and they deposit casts that form towers above the water level.

The researchers found each earthworm returns to the same deposit casts repeatedly for air and over time the tower becomes a mound over time.

The giant earthworms that create the surales are a previously undescribed species that belongs to the genus Adiorrhinus (pictured) and can grow up to five feet (1.5 metres) long

The mounds (pictured) are thought to be mainly formed by the action of the earthworms but flooding and plant roots also play a role, according to the researchers

When these mounds are close together the basin between them can become filled in by the worm's excrement to join together to form some of the largest features.

The roots of shrubs growing nearby and the flooding of the landscape itself play an additional role in helping to build up these mounds.

Professor Doyle McKey, an ecologist at the Centre for Functional and Evolutionary Ecology in Montpellier who was one of the authors of the research, said the surales can cover a far wider area than had ever been suspected.

The features had been little studied and their role in the delicate wetland ecosystems is poorly understood.

Satellite pictures showed the surales covered an expanse of up to 26,000 square miles. The pattern of mounds can be clearly seen from space (pictured) but their formation has never been studied in detail

Professor McKey said: 'We found that surales are much more frequent and widespread than was first suspected.

'We detected them in different zones within an area of about 75,000 square kilometres (29,000 square miles), an area greater than that of the Republic of Ireland.'

Writing in the paper, the researchers said the Andriorrhinus earthworm, which is a previously undescribed species, plays a key role in the tropical wetland ecosystem of the Llanos.

They said: 'The foraging behaviour of earthworms thus seem to produce the key mechanism leading to the formation of surales landscapes.

'Much remains to be learned about the ecological strategy of Andiorrhinus and its feeding and burrowing behaviour.'

In some cases the mounds join together to form labyrinth-like structures. The researchers say these mounds form a key part of the delicate wetland ecosystem across the Llanos wetlands (pictured)