Sisyphus is probably more famous for his punishment in the underworld than for what he did in his life. According to the Greek myth, Sisyphus is condemned to roll a rock up to the top of a mountain, only to have the rock roll back down to the bottom every time he reaches the top. The gods were wise, Camus suggests, in perceiving that an eternity of futile labor is a hideous punishment.
There are a number of stories—ones which are not mutually exclusive—that explain how Sisyphus came to earn his punishment in the underworld. According to one story, Zeus carried off Aegina, a mortal woman who was the daughter of Asopus. Sisyphus witnessed this kidnapping in his home city of Corinth. Sisyphus agreed to inform Asopus as to who had kidnapped Aegina if Asopus would give the citadel at Corinth a fresh-water spring. In making this deal and bearing witness against Zeus, Sisyphus earned the wrath of the gods while earning earthly wealth and happiness for himself and his people.
Another story tells how Sisyphus enchained the spirit of Death, so that during Death's imprisonment, no human being died. Naturally, when the gods freed Death, his first victim was Sisyphus. It is also said that Sisyphus told his wife not to offer any of the traditional burial rites when he died. When he arrived in the underworld, he complained to Hades that his wife had not observed these rites and was granted permission to return to earth to chastise her. Once granted this second lease on life, Sisyphus refused to return to the underworld, and lived to a ripe old age before returning to the underworld a second time to endure his eternal punishment.
Camus identifies Sisyphus as the archetypal absurd hero, both for his behavior on earth and for his punishment in the underworld. He displays scorn for the gods, a hatred of death, and a passion for life. His punishment is to endure an eternity of hopeless struggle.
We are not told how Sisyphus endures his punishment in the underworld: that much is left to our imagination. What fascinates Camus is Sisyphus's state of mind in that moment after the rock rolls away from him at the top of the mountain. As he heads down the mountain, briefly free from his labor, he is conscious, aware of the absurdity of his fate. His fate can only be considered tragic because he understands it and has no hope for reprieve. At the same time, the lucidity he achieves with this understanding also places him above his fate.
Camus suggests that Sisyphus might even approach his task with joy. The moments of sorrow or melancholy come when he looks back at the world he's left behind, or when he hopes or wishes for happiness. When Sisyphus accepts his fate, however, the sorrow and melancholy of it vanish. Camus suggests that acknowledging "crushing truths" like the eternity and futility of his fate is enough to render them less crushing. He refers to Oedipus, who, having suffered so much, is able to "conclude that all is well."