Mima mounds are small hills (about 2 m tall, about 10 m across) of unknown origin that occur in groups of hundreds or thousands in Washington State and other places. The most famous group of these mounds are at Mima Prairie near Olympia, Washington. I haven’t been at this site, but I have been at a smaller group of Mima mounds in the Channeled Scablands of Eastern Washington.
The Washington Department of Natural Resources has released incredibly detailed topographic models of Mima Prairie:
This image was created using LIDAR, which is sort of like RADAR, except it uses laser light rather than radio waves. LIDAR can see through vegetation (if you want it to; it depends on how you set it up and process the data) and can produce an image with accuracies of less than ten centimeters.
The origin of Mima mounds is uncertain. Common explanations include glacial or periglacial (i.e. near-glacier) processes, gophers, earthquakes, and Indian burial mounds.
HT: The Seattle Times (via my sister)
Wikipedia: Mima mounds
Grace and Peace
P.S. The U.S. Forest Service has a good overview of the use of LIDAR for natural resource investigations. The following image shows not only the ground surface, but the forest canopy: