Plato, Phaedo 112e ff (trans. Fowler) (Greek philosopher C4th B.C.) :
"[The] streams are many and great and of all sorts, but among the many are four streams, the greatest and outermost of which is that called Okeanos (Oceanus), which flows round in a circle, and opposite this, flowing in the opposite direction, is Akheron (Acheron), which flows through various desert places and, passing under the earth, comes to the Akherousian (Acherusian) Lake . . . The fourth river issues, it is said, first into a wild and awful place, which is all of a dark blue color, like lapis lazuli . . . Opposite this the fourth river issues, it is said, first into a wild and awful place, which is all of a dark blue color, like lapis lazuli. This is called the Stygios (Stygian River), and the lake which it forms by flowing in is the Styx. And when the river has flowed in here and has received fearful powers into its waters, it passes under the earth and, circling round in the direction opposed to that of Pyriphlegethon, it meets it coming from the other way in the Akherousian Lake. And the water of this river also mingles with no other water, but this also passes round in a circle and falls into Tartaros opposite Pyriphlegethon. And the name of this river, as the Poets say, is Kokytos (Cocytus). Such is the nature of these things.
Now when the dead have come to the place where each is led by his genius (daimon), first they are judged and sentenced . . . Those who are curable, but are found to have committed great sin . . . these must needs be thrown into Tartaros, and when they have been there a year the wave casts them out, the homicides by way of Kokytos, those who have outraged their parents by way of Pyriphlegethon. And when they have been brought by the current to the Akherousian Lake, they shout and cry out, calling to those whom they have slain or outraged, begging and beseeching them to be gracious and to let them come out into the lake; and if they prevail they come out and cease from their ills, but if not, they are borne away again to Tartaros and thence back into the rivers, and this goes on until they prevail upon those whom they have wronged; for this is the penalty imposed upon them by the judges."