Styx

The River Styx encircled the underworld nine times, forming an impassable barrier between the living and the dead. The ferryman Charon transported the souls of the deceased across its dark waters in exchange for a coin — which is why the Greeks placed an obol in the mouth of the dead.

The Styx held a unique power: oaths sworn upon its waters were absolutely unbreakable. Even the gods were bound by such vows, and any deity who swore falsely on the Styx was punished with nine years of exile from Olympus. Zeus himself used the Styx to enforce divine promises.

The river was also famous for its role in the story of Achilles. His mother Thetis dipped the infant Achilles in the Styx to make him invulnerable. The magical waters rendered his body impervious to weapons — everywhere except the heel by which she held him, creating his fatal weakness.