The mystery of the earth's "soil pimples" remains unsolved after a century

Gardening · Published 19 October 2026 Swipe more like this →

In short: Hundreds of bizarre, dome-shaped hills dot the prairies of the American West. Despite over 100 years of scientific digging, no one can agree on how they formed. Are they the result of ancient floods, seismic shifts, or the tireless work of tiny rodents?

The Mima mounds are a striking geological phenomenon found across the western United States and parts of South Africa. These mounds, which can reach up to seven feet tall and 40 feet across, appear as rounded piles of loose, unstratified sediment. Despite being a notable sight across the Mima Prairie in Washington State, these formations exist throughout almost all territories west of the Mississippi River, sparking a hundred-year argument regarding their beginnings.

Theories on their formation are as diverse as the landforms themselves. In 1913, geologist J. Harlan Bretz suggested they formed from "suncups"—depressions in snow and ice that collected sediment. Other hypotheses include ancient floods, seismic ground shaking from earthquakes, and the shrinking and swelling of clays. However, the most prominent theory involves pocket gophers. In 1942, a biologist first suggested these rodents built the hills while burrowing for roots, noting that the mounds often appear in equidistant patterns consistent with gopher territories.

The gopher theory faced significant pushback, notably from glaciologist A.L. Washburn in 1988, who pointed out that some mounds contain rocks too large for a gopher to transport. Furthermore, many mounds lack active gopher populations today. In 2013, a study by Manny Gabet used computer modeling to show that random gopher burrowing over hundreds of years could indeed create mounds matching the shape and height of those in California. Yet, even with this data, officials at the Mima Mounds Natural Area Preserve remain skeptical, as the process is so slow it is nearly impossible to observe in a human lifetime.

This enduring mystery highlights the complexity of geomorphology. Because similar mounds appear in different climates and ecological conditions—such as the "fairy circles" of South Africa—it is unclear if all such landforms share a single cause. Whether they are products of "bioturbation" (biological activity) or purely natural forces, the Mima mounds remain one of the world's most bizarre and persistent geological puzzles.

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