46.8975°N, 123.0514°W.
The Mima Mounds are a field of low circular earthen domes covering roughly twelve square kilometres of open prairie south of Olympia in Thurston County, Washington State, the type-site for a landform now recognised across western North America, southern Africa, and parts of South America under several local names. Each individual mound is two to three metres high and four to twelve metres across at the base, regularly spaced at intervals of three to five metres, and composed of well-sorted gravelly silt sitting directly on glacial outwash deposited by the late Pleistocene Vashon advance of the Cordilleran ice sheet roughly 16,500 years ago. Hypotheses for their origin include burrowing pocket gophers (the current mainstream view, advanced by Walter W. Dalquest and Victor Scheffer in the 1940s), seismic resonance shaking from regional earthquakes, periglacial frost-heave on then-permafrost soils, vegetation self-organisation similar to the fairy-circle model, and pre-contact Native American earthworks. None of these explanations is fully accepted by the current literature. The Mima Mounds are protected within the Mima Mounds Natural Area Preserve administered by the Washington Department of Natural Resources.
From the boardwalk they look like a frozen sea.