Way back about 27,000 years ago, a sinkhole from a collapsed limestone cave filled up with water to create a pond with steep sides. Mammoths, mostly the Columbian mammoths, slid down the sides to get a drink and then could not get out again. This occurred actually over thousands of years as the pond eventually filled up with silt that covered the dead animals. Eventually, it became a mudhole where mammoths and other animals used it to cool off, packing down the mud into the old sinkhole.
Eventually the soft limestone surrounding the mudhole weathered away, leaving the denser mud. When a bulldozer started preparing the ground for development, mammoth bones and teeth were discovered. Drill holes have determined that there are 65 feet of bones, while only the top 25 feet have been excavated. Nearly all the bones have been left in place, so you can see the positions of the dead animals. A building was built around the site to preserve it and allow research to continue with the University of South Dakota and volunteer groups.
It is really a neat place. All of the bones are from 20-35 year-old males, with the assumption being that like elephants, the females and young were in herds and knew better than to drink from the pond.
These next two photos show the most complete mammoth skeletons found.
On the wall, you can see the relative size of the various types of mammoths and modern elephants.
There is a small exhibit hall next door where they have reconstructed some of the mammoths and an example of a dwelling constructed entirely of mammoth bones and skin, as has been found in northern Europe.
And finally, as I left, I noticed outside the sifting area where they put all of the dirt that has come from digging so they can sift it for tiny bones and plant material.
I really enjoyed this place and bought a few booklets in the gift shop. I don't mind buying things when the organization is non-profit. Oh, and you can sign up for educational programs here, lasting from a day to a week or more!