This mystery might be cool to use in a class about hypotheses generation.

(Click on image for full size).

Specifically: “Mima mounds ( /ˈmaɪmə/) is a term used for low, flattened, circular to oval, domelike, natural mounds found in the northwestern United States, Idaho, Oregon, and Washington, that are composed of loose, unstratified, often gravelly sediment that is an overthickened A Horizon. These mounds range in diameter from 3 to more than 50 m; in height 30 cm to greater than 2 m; and in density from several to greater than 50 mounds per hectare. Within the northwestern United States, they are typically part of what is commonly known as hog-wallow landscape.” (wiki)

See all the hypotheses here. (Admittedly, I’m partial to the thought of busy gophers moving tons and tons of soil!)