The bones in the site date to approximately 26,000 years ago, and were only uncovered in 1974. In that year, a construction company was commissioned to work on a new housing development on the southern edge of Hot Springs. As they were digging on a particular hill, they began to find some odd bones, which were soon identified as belonging to a mammoth. The land owner, thankfully, agreed to allow scientists to investigate the area, and more and more bones were discovered. Once the significance of the site was realized, it was saved from becoming a housing development and was turned into a museum, and excavation continues today. Over 60 mammoths have been found so far, as well as a few other animals like prehistoric camels, bears, wolves, llamas, and rodents.
Even when you first walk in, you can see an impressive amount of bones. It's a big site! Some bones have been removed for study, but many others have been left where they were found so visitors can see them in their original positions and locations. The portions of the rock that have been left intact are covered with a prolific layer of bones, which really helps you appreciate the amount of mammoth skeletons that have been found here.
I'm glad you asked. Several geologic and environmental factors all came together to create the Mammoth Site, but it all started with a cave far below the earth's surface. The cave eventually collapsed from the weight of the overlying rock, creating a sinkhole. Soon, a spring formed below the sinkhole and warm, mineral-rich water bubbled up to the surface, creating a small pond. This pond appeared attractive to the diverse wildlife that roamed the surrounding plains, but it held a hidden danger. The surrounding rock was composed of shale, and the walls encircling the pond were rather steep and slippery. If the water level in the pond was low, a mammoth would have to lean pretty far down to be able to reach it. And if that mammoth slipped and fell in to the pond? Bad news. The walls of the pond are already very smooth and steep, and mammoths, like their modern elephant relatives, have flat feet that are not at all suited for climbing. So any mammoth (or other animal) that fell in would never come back out - they would die of exhaustion or starvation in the pond. The pond existed over a period of time that lasted between 350 and 700 years, and many mammoths would fall to the same trap over that time. Their dead bodies sunk to the bottom of the pond, where they were gradually covered with sediment and buried. Eventually, however, the spring dried up and the muddy lakebed could trap only footprints. The sediments were cemented with calcite and turned to rock. This rock was more resistant to weathering than the surrounding shale, so as the shale eroded away, the former pond bed became a hill.
The moral of the story? Boys are dumb.
Anyway, getting back to the tour, this is the bone that lets researchers know the sex of a mammoth from its skeleton - the pelvic bone. Females have a much wider space in the pelvic bone than males do, because females need to have enough room for a baby to pass through that opening and males, obviously, do not. So you can determine a skeleton's sex by taking measurements of its pelvis.
If you take the elevator down from the museum, you can actually visit the lab where people are busy at work analyzing the bones that have been found at the site. A row of windows lets you peek in and see what the scientists are doing. I was there late in the day, so there was only one person there, who was operating some kind of loud machinery that (I think) was cleaning bits of rock off the bones. There are work stations right on the other side of the windows, and some of the other workers had posted notes on the windows to let visitors know what they were doing. One note beside a large tray of small rocks and a high-powered magnifying lens said that the worker here had been picking through the debris looking for bone fragments from mice, rabbits, and other small mammals. Sounds like quite a job!