You've heard of Styx, right? The ancient river that kept people inside Hades. But it wasn't only a river. Styx is also the name of the oldest daughter of Oceanus and Tethys. She aided Zeus in his battle with the Titans. Zeus honored her by by making vows made in Styx's name irrevocable even for gods.
Another note of interest: As of July 2013, Styx became the name of one Pluto's moons. The other moons--Charon, Nix, Hydra, and Kerberos, also have names from Greco-Roman mythology.
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