We saw a very large, slow moving craft.I and my wife were driving back from Pocatello. We were about to the raft river area on Interstate 84. I saw what I thought at first was perimeter lights to a house or a compound. The lights were on the right hand side of the road and seemed to be on top of a hill in the distance. However, after seveveral minutes, the lights stayed at their relative distance and hight. I then pointed the lights out to my wife. She thought, as I did at first, that the lights were perimeter lights or possibly lights from night skiing. Pomerel mountain was too far distant for this to be the case. We then both noticed that we were gaining on these lights. As we got closer, we could make out about 15 to 20 lights. Our speed was 75 mph at that time. When we got to the top of the incline we were traveling on, we at first lost sight of the lights. We then noticed the lights on the left side of the highway. It had crossed the highway. We could see it in a valley across from us, and we slowed to its speed, about 67-68 mph. It was about 500 to 1000 feet off the ground. At one point I could see under it and it had the outline roughly of a triangle with the courners rounded. It lit the sky above it, and it was dark grey beneath. It was at that point about less than a mile from us. I guessed it size to be about 200 to 300 feet across the back of it and about 150 to 200 feet from front to back. It was about 30 to 50 feet thick. The lights on top were all of the same color, about that of a mecury vapor lamp. We also noticed ,after the fact, that there are many farm houses and dairies in this area that this object was over during the time it was on the left of the highway. None of those lights were on as it passed over that area. It finally passed behind a mountain range. As it did, it lit up the profile of one of the montains. It was traveling south towards Nevada. My wife saw the same object, in the same place, at the same time, 2 weeks laiter to the day.