The Mima Mounds Natural Area Preserve is a designated National Natural Landmark near the Capital State Forest and Maytown. The 3′ to 6′ high mounds themselves are kind of cool, large, rounded hills. The Mima mounds appear in parts of Washington, Oregon, Texas, Arkansas, Louisiana and California, and also in Kenya, Mexico, Canada, Australia and China. Scientists don’t really know what caused them, but theories include pocket gophers, wind blowing around vegetation, seismic activity, shrinking & swelling clay and, my personal favorite, space aliens.
Fair warning, this park requires a Discovery Pass which can be purchased online here.