Born from chaos, Tartarus is the lowest point in the world. Hesiod writes that it would take a bronze hammer ten days to fall to Tartarus, if dropped from the earth. It is surrounded by a wall of bronze, and is guarded by the Hecatoncheires.
In early myths, Tartarus is a prison for defeated gods. Cronus and the Titans were imprisoned there. Later, Tartarus becomes a place of punishment. Sinners, such as Tantalus, were placed there and tormented for eternity. By the first century BC, Tartarus had become the equivalent of the Christian hell. When Aeneas visits the underworld, he sees Rhadamanthys judging the sins of man, sending the good to Elysium and the wicked to Tartarus.
Homer. The Iliad.
Virgil. The Aeneid.