The Underworld, also known as Hades, was hidden deep in the earth and was the kingdom of the dead. It was the location of the afterlife in Greek mythology. It is not to be confused with the god, Hades, its ruler.
Geographically, the Underworld was considered to have been surrounded by five rivers: the Acheron (river of woe), the Cocytus (river of lamentation), the Phlegethon (river of fire), the Styx (river of unbreakable oath by which the gods took vows and hatred), and the Lethe (river of forgetfulness). Past the rivers, a diamond gate, guarded by Cerberus, formed the entrance to the kingdom. Deep within the kingdom, lay Hades' vast palace, full with guests.
Upon death, a soul was led by Hermes near the entrance of the underworld, where the ferry awaited to carry it across the Acheron. There was a single ferry run by Charon, the boatman who took the souls across the river. Only those who could pay the fare with coins placed on their lips when buried, were granted passage; the rest were trapped between two worlds.
After the boat ride, the souls entered through the gates; Cerberus allowed everyone to enter, but none to leave. The souls then appeared before a panel of three judges, Rhadamanthus, Minos, and Aeacus, who passed sentence based on their deeds during their previous life. The souls who were good went to the Elysian Fields, while the others were singled out for special treatment; Sisyphus and Tantalus are two examples of souls that were sentenced to be tormented for eternity.