Simulated Gophers Explain Mysterious Mima Mounds

The first European explorers thought they were American Indian burial mounds, but found only dirt and pebbles inside when they sliced one of the mounds in half.
Mima mounds 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.