The Underworld

The Underworld is the final resting place of the dead and is ruled by the Olympian God Hades

River Styx
Also known as the Pit of Death. This river carries the dead to the Underworld where they will spend the rest of eternity in either the Elysian Fields if they were good or Tartarus if they were evil. In the episode of the animated series "Hercules and the River Styx" it is revealed that the river is also a Goddess that only reveals herself to those who wield Poseidon's Trident. It is also revealed that the river marks the border of Hades' domain. Technically Styx is in fact a nymph, and will live for as long as her river flows.

Tartarus
Tartarus is the deepest, darkest and most sinister area of the Underworld. It is here where the souls of the wicked and evil are sent and it is also the place where Zeus imprisoned most of the Titans and a giant Cyclops after he ended their reign of destruction.

It is a dark and ominous dungeon-like place located deep beneath the ocean, a few jagged rocks can be seen rising from the sea where Tartarus lies. There is also a small island filled with jagged mountains where the Titan Prometheus is imprisoned and each day a giant eagle comes to eat his liver.

Elysian Fields
The final resting place for the good and the heroic. It is a paradise filled with beautiful valleys and temples. This part of the Underworld is protected by Zeus, but Hades attempted to win it from Zeus in a bet in the episode "Hercules and the Driving Test", but he lost the bet however.

Trivia

 * In the Juniper Lee's Adventures Series episode The Last Stand. The evil galactic emperor Saw Boss is killed and imprisoned in the Underworld for all eternity. though Hades is shocked to see he was able to keep his body.