Image caption These Mima mounds in Washington State cover a huge area

The mystery of one of the strangest landscape features on the planet - Mima mounds - has been solved, scientists say.

These geological anomalies are circular hillocks that cover great swathes of land. But scientists have been puzzled about what causes them.

Now new research suggests that tiny burrowing animals are their architects.

The findings will be presented at the American Geophysical Union Fall Meeting in San Francisco.

Until now, nobody really knew how they formed Dr Manny Gabet, San Jose State University

Mima mounds, which measure up to 2m (7ft) in height and 50m (160ft) in diameter, are found all around the world. However, they are most common in North America.

In some areas, they can number in their millions, stretching for many kilometres across the landscape.

Lead researcher Dr Manny Gabet, of San Jose State University, told BBC News: "The big mystery surrounding Mima mounds is that, until now, nobody really knew how they formed.

"Over the past couple of hundred years, people thought they might be Native American burial mounds, or they were caused by earthquakes or glaciers. Some people even suggested extraterrestrials."

Image caption It takes many generations of gophers hundreds of years to make a Mima mound

Now though, Dr Gabet says he is certain that gophers have created the mysterious mounds.

Using a computer program, the researchers analysed how the rodents move soil as they burrow.

They found that in areas prone to waterlogging, the gophers gradually shift tiny amounts of earth upwards to try to stay dry.

Over hundreds of years, though, as many generations of gophers repeat this process, these minute piles of soil grow into the large structures.

Dr Gabet said: "I developed 'digital gophers' and had them behave like they do in real life, and to my surprise Mima mounds just started to form in this virtual landscape.

Image caption Mima mounds created by "digital gophers" are almost identical to the real features

"The [computer] model results look so similar to the mounds in every way - not just the dimensions, but also the way they are packed and how many you get per area."

He added: "It replicates the real-life situation almost perfectly."

But gophers are only found in America, while Mima mounds are found in every continent apart from Antarctica.

However, Dr Gabet said that other subterranean mammals, such as moles, could be responsible for these.

"Mima mounds in the US are infested by gophers," he said.

"A good place to start would be to dig into these mounds [in other countries] and see what kind of critters are living inside of them."

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