This area of expansive prairie-covered mounds south of Tumwater has been a scientific curiosity since 1841, when Capt. Charles Wilkes of the US Exploring Expedition made a special trip to see it. Theories on the creators of these abundant earthen mounds range from gophers to glaciers. Many scientists believe they may have been created by freezing and thawing conditions when the last ice age glaciers retreated from the area some 10,000 to 15,000 years ago. In 1967 they were designated a National Natural Landmark, and in 1976 the state purchased a 445-acre tract as a Natural Area Preserve; the paved Mima Mounds Natural Trail (.5mi) leads through the mounds; a 2mi loop extends south. The mounds support a native prairie grassland which includes wildflowers, butterflies and birds.