Final answer:
Achilles was dipped in the river Styx, a river from Greek mythology that signified the boundary between the living world and the afterlife.
Step-by-step explanation:
The river where Achilles was dipped is the Styx. Achilles was a figure from Greek mythology whose mother, Thetis, dipped him into the Styx, which was believed to confer invulnerability. Unfortunately, she held him by his heel, which was not touched by the waters and thus remained his only vulnerable spot, known as Achilles' heel. The river Styx was central in Greek mythology as the boundary between the living world and the underworld, and was presided over by the ferryman Charon, who transported the souls of the newly deceased across its waters.