My friend and I (ages 55) driving west on Hwy 24 about 15 miles east of Hill City briefly saw a large silver cylinder about 1/3 mile ahead west above the highway, rise and fall and fly north east about about 50 miles per hhour at about 50 feet above ground. It appeared to be a cylinder about the size of a 747 fuselage, but wider in girth and it was a hollow-looking cylinder with no wings, no windows, and no visible propulsion engines. My friend and I described it separately, rather confirming each others interpretation. It left no trail. It was travelling probably 50 miles per hour. This was Friday, June 22 at about 2 PM central time, 2007. It was a cloudless sky, however we could see the craft only briefly between hills from the road. I sighted it first for about 8 seconds, and asked my friend to observe it while I drove, he also saw it for about 10 seconds before it disappeared behind hills. I would estimate