When driving north on I-5 just north of Seattle we noticed a group of persons standing on the side of the freeway pointing and looking upward to the sky just East of the Freeway at an elevation of about 3000 feet. We pulled over and joined the group to stare amazingly at the aircraft. It was about 3-400' feet long, shaped like the Goodyear Blimp, was hovering motionless with no sound (no engines could be seen anywhere on the craft), and it had brilliant multicolored lights from top to bottom. The lights were going on and off, moving in a clock-wise pulsating fashion from end to end. Truly an amazing sight. After staring for about 5 minutes the craft, without a whisper of any sound, shot straight East at a low altitude and disappeared in less than 3 seconds. We all looked at each other in disbelief, got in our cars and headed North on 1-5.