For some reason, when I first wake up in the morning is when my mind tends to have insightful -- but mostly crazy -- thoughts. So, let the debunking... erhm... discussion begin!
What if the Mima Mounds in western Washington are the site of some ancient hugelkultur beds? Perhaps their fertility has long since run through, so they maintain their structure but can currently only support prairie. Or perhaps they were designed that way. There is a lot of gravel in the mounds, so perhaps the first peoples in the region were attempting to grow prairie plants that were more suited to a warmer climate?
I'm probably crazy and way off base, but for some reason one of the first thoughts in my head this morning was "maybe the Mima Mounds are really ancient hugelkultur beds."
I think gophers did it. There's no evidence of the sort of human population that would have been required to maintain such a large complex. Human artifacts and bodies have not been found within the mounds. The uniformity of distribution suggests a territorial animal. Each colony would need enough grazing space. I imagine that this would be a great place to be a hawk with all the gopher meat available.
Or, they might be burial mounds. Perhaps some unknown race of space monkey revered the gophers as gods and built huge altars in which to dispose of carcasses. To test this theory, I'll find out if they have found any gopher skelatons inside the mounds. If so, then I am right. !!! That's how it works. I've seen those UFO shows. If it can't be conclusively proven wrong, then it must be the truth. The existence of a yokel who has seen, heard or dreamt "strange things" would lend further support to my theory.