It's one of those things that teachers in Washington state use to hook kids on science, alongside salmon runs, hydropower plants and those weird sea stars with way too many arms. Mima mounds, I diligently learned, are 6-foot-tall, 30-foot-wide domes of dirt that cover the open prairie on Washington's Puget lowlands. They are so large and uniform, it's hard not to feel they must be man-made. But nobody actually knows who or what made them. They are a scientific mystery, I learned.