A prairie of unanswered questions
The Mima Mounds Natural Area Preserve protects a bizarre landscape that has puzzled scientists since the first formal investigation in 1841. Located on a glacial outwash plain in Thurston County, Washington, the preserve is covered in thousands of dome-shaped mounds of earth. These mounds are remarkably uniform, typically reaching heights of one to two meters and diameters of eight to twelve meters. They are composed of loose, black silt and gravel, sitting atop a hardpan of denser, stratified gravel left behind by the Vashon Glaciation around 16,000 years ago.
The sheer number and ordered and non-geometric spacing of the mounds look unusual. Designated a National Natural Landmark in 1966, the 756-acre preserve protects one of the finest and most accessible examples of this phenomenon. The mounds are not burial sites, a theory disproven by early excavators who found no human remains or artifacts. The central question remains: what process could create such a vast and regular pattern of earthen bumps? For nearly two centuries, investigators have proposed dozens of explanations, but no single theory has achieved universal acceptance.
Gophers, earthquakes, or glaciers
Three main hypotheses are the main scientific explanations. The longest-standing biological explanation, formally proposed in the 1940s by biologists Walter Dalquest and Victor Scheffer, is the fossorial rodent theory. It posits that generations of pocket gophers (Thomomys talpoides) built the mounds over centuries. In this model, gophers push excavated soil towards a central, dry point in their territory. The waterlogged soil in the low-lying areas between mounds naturally limits their burrowing, enforcing the spaced-out pattern. Computer models have shown that gopher activity over 500-700 years could indeed produce fields of mounds identical to those observed.
A leading geological explanation involves seismic activity. First proposed by Andrew Berg in 1990, this theory suggests that strong, sustained ground shaking from major earthquakes could cause loose surface soils to self-organize into heaps. Berg demonstrated that vibrating a layer of silt on a board could create miniature mounds, lending experimental support to the idea.
Other theories look to the region's icy past. One early idea from geologist J Harlen Bretz in 1913 suggested the mounds are remnants of sediment that collected in "sun cups"—depressions melted into a decaying ice sheet. A more recent flood hypothesis connects the mounds to a massive outburst flood from a glacial lake on ancient Mount Rainier, which deposited volcanic rock and other sediments across the prairie. This story aligns with oral traditions of the local Chehalis people, which describe a great flood transforming the landscape.