In Greek mythology there were five rivers in the Underworld. You likely know of the river Styx. (Name of an awesome rock band as well.) The other four rivers were the Lethe, Archeron, Phlegethon, and Cocytus. These rivers were associated with various aspects of the Underworld, either gods or emotions. For example, the river Lethe is the river of oblivion, causing the dead to forget their earthly life. The Phlegethon is a river of fire, punishing the dead. The Cocytus is a river of wailing and lamentation.
These rivers show up in the Divine Comedy, but Dante gives the mythology his own twists. In Canto XIV, Vigil and the Pilgrim come to the fiery, blood-filled river Phlegethon. There Vigil explains to the Pilgrim the source of all the rivers running through hell. Virgil describes a giant old man who is encased within Mount Ida on the island of Crete. The "Old Man of Crete" isn't to be taken literally, Dante is using it as a symbol.
Dante mixes mythical and Christian symbols in describing the Old Man of Crete. The Old Man of Crete represents human history, similar to the statue described in Daniel 2 representing four kingdoms.
The twist Dante adds is that the Old Man of Crete is crying, and his tears flow through the fissures of the ground to create the rivers of hell. Virgil describing this:
"Every part of him, except the gold, is brokenThe Cocytus, if you haven't guessed, flows to the lowest level of hell where we'll find Lucifer being punished. Also, the Lethe is missing in Virgil's description here, but it is encountered later in the Comedy.
by a fissure dripping tears to his feet,
where they collect to erode the cavern's rock;
from stone to stone they drain down here, becoming
rivers: the Acheron, Styx, and Phlegethon,
the overflow down through this tight canal
until they fall to where all falling ends:
they form Cocytus. What that pool is like
I need not tell you. You will see, yourself."
The theological point of interest here, for me at least, is how the tears of the Old Man are taken to represent the tears of fallen humanity down through the ages.
Basically, the tears of the Fall create the rivers of hell.
I find something beautiful, tragic, sad, and scary in that vision.
Think of all the tears we have caused. Think of all the tears caused by hate, abuse, war, oppression, violence, hurtful words, and broken promises.
How many tears have you caused? How many people have you made cry?
All those tears, none of them lost, gathering and flowing into the terrible judgment of God.