It's hard to imagine a time when Muhammad Ali was the ultimate underdog.

But 50 years ago today, when Ali, known then as Cassius Clay, stepped into the ring with heavyweight champion Charles "Sonny" Liston, pretty much everyone thought the younger, less experienced Clay would be walloped.

Clay shouts at Liston during the weigh-in. | (Bettmann/CORBIS)

But Clay did not go down. Instead, Liston withdrew from the bout after the seventh round, citing a shoulder injury. Clay's subsequent celebration — jumping around the ring, shouting, "I'm king of the world!" and being restrained by his trainers — has become the stuff of legend, turning a relative unknown into the fighting force known as Muhammad Ali.

(Whether the fight was legitimate is another story altogether. The Washington Times acquired documents from the FBI suggesting the mob may have leaned on Liston to throw the fight.)

Let's take a look back at the fight that shook the boxing world:

Clay whips a right to Liston's head. | (AP Photo)

Liston misses a long left. | (Bettmann/CORBIS)

Liston drives a mean right to Clay's chin. | (Bettmann/CORBIS)

Clay throws a straight left to Liston's face. | (AP Photo)

Liston ducks low and weaves to escape a punch. | (AP Photo/stf)

Bobbing and weaving. | (AP Photo)

Clay's handlers hold him back after he defeats Liston, becoming the new heavyweight champion of the world. | (AP Photo)

Muhammad Ali shouts, "I am the greatest," as he leaves the ring. | (AP Photo)