Thetis (THEE-tis or THEH-tis). Best known of the Nereids, the fifty daughters of the Old Man of the Sea; mother of Achilles, the Greek champion in the Trojan War. When Theseus was challenged by King Minos to prove himself a true son of the sea-god Poseidon, he dove into the waves to retrieve the king's signet ring. He found the ring and an underwater palace as well, where Thetis (or her sister Amphitrite) presented him with a crown.