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After the war between the Titans and the Olympians, Atlas was punished. Zeus forced him to hold the heaven on the western edge of the world. Later, the story about him changed and it was said that he held the whole world on his shoulders. Mostly in modern times, he is shown holding a globe of the world.
Heracles came to Atlas and asked him to for help with one of his Labours. He wanted Atlas to get the "Golden Apples" for him as only an immortal could take them. Atlas agreed, but only if Heracles would hold up the sky. Atlas was pleased to be free from its weight but Heracles tricked him into holding it again.
Perseus also came to Atlas for help after killing Medusa. Atlas, who was tired of holding the heavens, asked Perseus to allow him to see Medusa's head. Perseus took out Medusa's head and Atlas turned into a mountain.