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.
According to Hipparcos, the New Reduction (van Leeuwen, 2007), the estimated distance to the star is about 70 parsecs or 228 light-years.
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).