The mounds range from 3 to 8 feet in height and up to 30 feet in diameter.
A 'Discover Pass' is required for vehicle parking at this site.
The prairie ecosystem here is categorized as a rare, fire-dependent habitat.
Archaeological surveys have found no evidence of human construction in the mounds, supporting natural formation theories.
The preserve contains a specific lichen and moss soil crust that is highly susceptible to damage from foot traffic.
The site was designated a National Natural Landmark in 1966 due to the unique geological puzzle it presents.
Mima Mounds Natural Area Preserve is a 637-acre landscape defined by thousands of low, circular, earth-filled mounds of unknown origin. Located in Thurston County, Washington, the site features a paved loop trail and an unpaved interior loop that navigate through the prairie ecosystem. Geologists and biologists have debated the formation of the mounds for over a century, with theories ranging from seismic activity and glacial outwash to pocket gophers. The site is a rare remnant of the once-extensive South Puget Sound prairie, now a protected habitat for native wildflowers and grasses. Visitors can access an observation deck that provides a clear vantage point of the topographical patterns. The area is maintained by the Washington State Department of Natural Resources to protect the fragile soil crust and native vegetation. It serves as an outdoor laboratory for ongoing geological research.
The raised wooden observation deck near the trailhead provides the best panoramic view of the mound topography.
Bring binoculars to observe the prairie birds and detailed patterns of the mounds from the observation deck.
Stick strictly to the marked trails to avoid damaging the sensitive prairie soil crust.
The area is fully exposed to the elements, so carry sunscreen and rain gear regardless of the forecast.
Stepping off the designated trails, which can cause long-term damage to the fragile prairie vegetation and soil structure.
Stay on marked trails at all times; no dogs, bicycles, or motorized vehicles permitted on the trails; carry-out all trash.
