A 600-acre preserve of glacial outwash prairie 25 km southwest of Olympia, covered by thousands of low domed mounds — typically 2 metres high and 8 metres across — whose origin has never been definitively explained. Theories range from prehistoric earthquake liquefaction to long-term gopher activity.
A short network of paved trails leads from the small visitor centre out across the prairie; a wooden tower lets you see the mound pattern from above. The prairie is at its most spectacular in May, when the wildflower display includes camas, balsamroot and the rare golden paintbrush. Free entry, day-use only.