The mounds were documented by Charles Wilkes in May 1841 during a global expedition. Wilkes speculated the mounds were burial sites of Native Americans, but after digging into a few of the features, no relics were found by his team.
The mounds, also described as hillocks, are situated on a plain of underlying cobble that is most likely a remnant of a glacial lake.
The origins of the word, mima (MY-ma), has been reported to be unknown, though it may mean, “newness”, under the language of the Chehalis people.
The mounds were once part of a larger stretch of a prairie ecosystem encompassing approximately 180,000 acres (73,000 ha), spanning from present-day Joint Base Lewis–McChord through Chehalis and into Oakville.
There are approximately 8-10 mounds per acre, measuring 1–7 feet (0.30–2.13 m) in height and having diameters 7–40 feet (2.1–12.2 m).