In 1989, inspection of the Lake Ilo dam found that critical maintenance was needed, and a notch cut into the spillway lowered the water about 7 feet so the work could be completed. The exposed lake bed revealed discoveries and mysteries of Plains Indian life in western North Dakota. A stone circle discovered on the lake bed led the Refuge manager to contact the North Dakota State Historic Preservation Office, and a University of North Dakota archaeological team recorded 13 prehistoric sites, including stone tools, waste flakes, hearth stones, and pottery shards, some representing campsites that had been undisturbed for more than 10,000 years. The stone circle is thought to be the remains of a Native American home, perhaps a bison-hide tent, one of the tipis that sheltered the highly mobile Native American hunters and gatherers who lived in this area. Stones were often used to anchor the hides that covered the poles. A hide tent requiring a stone circle of this size would have been approximately 22 feet tall, with 25 poles and a covering made of 37 bison hides, and would have weighed almost 800 pounds without the stones. Lodges this large were not practical until horses were available to move them sometime in the early 1700s, yet a 500-year-old roasting pit discovered inside the stone circle dates to the early 1500s, leaving unanswered questions about whether the large stone circle predates the horse and what it was used for, or whether the roasting pit came from an earlier occupation and the stones were added later. Many smaller stone circles created before the use of horses have been discovered on the Plains, and dogs were used as pack animals to transport these smaller tipis.