Hera's implacable fury was well-known, especially against the various extramarital lovers of Zeus and their children. She placed serpents in the cradle of Heracles, sent a hundred-eyed giant to watch over Io, and tried to prevent the births of Athena and Artemis. As punishment for the madness she imbued upon Heracles that caused him to kill his parents, Zeus chained Hera to Olympus with anvils tied around her ankles.