In antiquity, the River Acheron was the most prominent of the five rivers of the Underworld, and in some ancient texts it was said to be only slightly less important than the earth encircling river Oceanus.
The River Acheron was perceived to be a physical barrier between the Underworld and the mortal world, for mortals could not cross it to enter the Underworld, and the dead could not cross it to escape.
To this end, Hermes, Thanatos, or another Psychopomp, would bring the souls of the deceased to the bank of the Acheron, and Charon, the ferryman, would transport the souls across the river on his skiff. Transport was dependent upon payment, for during funeral rites coins would be left in the eyes or mouth, of the deceased.
Those who could not pay would be left to wander aimlessly along the bank of the Acheron, and could possibly give rise to ghosts in the mortal realm. It was also along the far bank of the Acheron along which Cerberus, the triple headed dog, would patrol.
