Nearing Burlington, CO predawn on Dec 8, 2007 the entire horizon lit up in a huge semi-circular flash of purpleWe did not actually see a UFO, however, my 3 children and I were driving west on I-70 through a snowstorm nearing the Burlington exit when suddenly there was a loud (ear-splitting) burst similar to the thunder that accompanies a lightning storm; the entire horizon lit up in a huge semi-circular flash of purple light. We wondered if it was an explosion, some power lines having fallen or something, but saw nothing afterwards such as fire or emergency vehicles. When we arrived at the Denny's shortly thereafter, the waitress admitted she had witnessed the exact same phenomenon.