Nature
For decades people were puzzling over a phenomenon of mysterious origin that stretched across every continent on the globe apart from Antarctica. Residents of Earth and scientists alike had started to pay attention to vast fields of meters-high circular mounds, hillocks that have since become known as “Mima mounds” – but nobody knew what was making them.
As is often the case with mysterious structures found in the Baltic Sea or up here on land, the puzzling phenomenon got the tin foil hats twitching, with ideas ranging from earthly processes like earthquakes to the extraterrestrial cop-out of alien interference. One leading theory, however, suggests that the Mima mounds are most likely the work of *checks notes* gophers.
Mima mounds
Mima mounds are hillocks made up of loose soil mixed in with other organic material. They’re typically circular to oval in shape and can be up to 2 meters (6.6 feet) tall with widths ranging from a couple of meters high to tens of meters wide in the most extreme cases.
They’re often found in vast clusters dotted across entire fields, with one of the most remarkable places to see them being the Mima Prairie in Thurston County, Washington. That’s where the name comes from, but Mima mounds are found all over the globe. So, where were they coming from?
What was making the Mima mounds?
There were a lot of theories floating around to explain the Mima mound mystery, but for scientists at San José State University (SJSU) in California, gophers were a prime suspect. An earlier 1987 study that used iron pellets to track the movement of soil at the hands of burrowing gophers revealed the impressive way in which they push the soil upwards instead of downwards as they dig.
Armed with that information, the SJSU team put together a computer model to recreate the movement of gophers and observe how it altered the landscape of a virtual world. As detailed in their 2014 paper, the resulting mounds bore a striking resemblance to the Mima mounds, both in their height, diameter, pattern, and frequency.
A halt in farming practices in the 1980s offered an opportunity to study the gopher’s digging in the real world, too. Untilled fields showed that over a period of 30 years they built mini Mima mounds ~25 centimeters (9.8 inches) in height, a rate that was mimicked by the virtual gophers.
According to the researchers, that means it would take around 500 to 700 years for gophers to build a Mima mound like those we see towering meters-high in Washington. For all their impressive digging skills, they don’t live that long, meaning the feat must be achieved across hundreds of generations.
Animal engineers?
So, mystery solved? Well… almost. According to geomorphologist Emmanuel Gabet of SJSU, the study's lead author, there’s no way to prove for absolute definite that the gophers are behind these Mima mounds that grow so slowly, and it might be that mounds in different places have different explanations.
"I would say they've got a strong case,” Robert Anderson, a geomorphologist at the University of Colorado, Boulder, told Science. "I think they're right. It's an example of how nature can organize itself into patterns that are striking."
Not convinced? Just check out the even smaller masterminds behind these huge towers that date back thousands of years, or the artisans stamping octagons across the seafloor.
Amendment 5/2/25: this article was updated to clarify that the gopher theory is only one, albeit a leading, possible explanation behind Mima mounds.
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