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.