In Greek mythology, Styx (; Ancient Greek: Στύξ [stýks]) is a deity and a river that forms the boundary between Earth and the Underworld, often called "Hades", which is also the name of its ruler. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the center of the underworld on a great marsh, which sometimes is also called the Styx. According to Herodotus, the river Styx originates near Feneos. Styx is also a goddess with prehistoric roots in Greek mythology as a daughter of Tethys, after whom the river is named and because of whom it had miraculous powers.