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Mima Mounds: The 15,000-Year-Old Mystery That Still Baffles Geologists
Thousands of 30-foot mounds across Washington, Oregon, and Idaho form a pattern that looks like a giant fingerprint. A century of study has not solved the Mima mounds mystery.
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A stretch of the American West looks like the Earth has goosebumps. Thousands of evenly spaced mounds, some 30 feet tall, cover hundreds of miles across Washington, Oregon, and Idaho. Scientists call them the Mima mounds.
The pattern is so uniform that aerial photos look like a giant fingerprint pressed into the soil. After more than a century of study, no one can say exactly how they formed.
Early theories suggested glaciers left them. Others pointed to gophers. One researcher in the 1950s argued that native peoples built them as burial sites.
A 2014 study in Geomorphology offered the strongest evidence yet. The mounds appear only in areas that experienced repeated freezing and thawing during the last ice age. The process, called frost heaving, pushes soil upward into organized patterns, much like the stone circles found in Arctic regions.