Mysterious Mima mounds likely made by burrowing rodents

Scientists may have solved the mystery of Mima mounds - one of the strangest landscape features on Earth - after a computer simulation suggested that the circular hillocks are made by tiny burrowing rodents.
Mima mounds, which measure up to 7ft in height and 160ft in diameter, are found all around the world. However, they are most common in North America.
Despite often numbering in millions within a single field, their origin has been a mystery, with proposed explanations ranging from glacial processes to seismic shaking.
Early explorers in the US believed them to be Native American burial sites.
"The big mystery surrounding Mima mounds is that, until now, nobody really knew how they formed," lead researcher Dr Manny Gabet, of San Jose State University in US, told BBC News.

"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," Gabet said.

Gabet is now certain that tiny burrowing gophers have created the mysterious mounds over hundreds of years.
Researchers used a computer programme to analyse 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.

"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," Gabet said.
"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.
"It replicates the real-life situation almost perfectly," he said.
But gophers are only found in America, while Mima mounds are found in every continent except Antarctica.
However, Gabet said that other subterranean mammals, such as moles, could be responsible for these.

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