As fires burn, smoke goes up the chimney. Some of that smoke condenses in creosote on the flue -- the inner surface of the chimney. Creosote, a hard tar-like substance, builds up over time. When a thick coat of creosote catches fire, you get a chimney fire -- a powerful and frightening inferno that can be as loud as a low flying jet.Chimney fires are often so hot that nearby walls and rafters burst into flame. The fire spreads so fast that firefighters often arrive too late to save any of the home.Not all chimney fires are loud roaring events, but quiet ones can be just as dangerous. The extremely high temperatures associated with chimney fires cause cracks to form in the flue and chimney.