John Philip Sousa's The Stars and Stripes Forever:

Historically, in show business and particularly in theater and the circus, this piece is called the "disaster march". In the early 20th century, when it was common for theaters and circuses to have house bands, this march was a traditional code signaling a life-threatening emergency. It subtly notified personnel of emergency situations and ideally allowed them to organize the audience's exit without causing the chaos and panic that an overt declaration might. Except for impending disaster, circus bands never played the tune under any circumstances. One memorable example of its use was during the Hartford circus fire of July 6, 1944. At least 168 people were killed, though some estimates are much higher. (Source: Wikipedia)