Achilles' mother was Thetis, a sea nymph in Greek mythology. She was the daughter of Nereus and Doris, and was married to Peleus, the king of the Myrmidons. Thetis was also associated with the river Styx, which was the boundary between the Earth and the Underworld. In order to make Achilles immortal, Thetis dipped him in the river Styx, but she held him by his heel, which remained vulnerable and became known as the 'Achilles TENDON'.