We were approaching the Bingham Copper Mine Pit and were on a limited access road. It was mid-morning and we were driving quite slow with the windows down and the radio off. As we started toward the top of the hill, a large, dark shadow came over the mountain to the west of us and covered the entire car. We initially thought it was a cloud, but the shade was much darker than what cloud can provide and the edges were crisp and defined in a circle. There was no noise, and when we looked out the windows up into the sky the disc shot over the car (as if startled?) and zipped across the valley to hover above the city below. It stayed motionless for about 15 seconds and then just disappeared. We didn't see it move off or anything-- it just wasn't there anymore.