The Underworld is a place where Hades lives. He is ruler, and keeps guard of shades - human souls - who board Charon's boat to the Underworld. For entrance on Charon's boat, they must pay one obol. Once getting off the boat, they file past Ceberus, a three-headed dog whose job is to keep souls from leaving. In the land of the dead, it is a gloomy place, with marshy grass.
Hades remarks that almost nothing grows there, mostly asphodel. Persephone sees this when going in the fields of asphodel, but likes the plant.
A bearded man with wings named Thanatos carries a scroll and helps gather the shades. Hypnos, who also helps in the Underworld, looks similar to Thanatos and Persephone guesses they are twins.
When Persephone heads down a steep slope towards Tartarus, the lowest level of the Underworld, she sees shades that are punished in a pool of "intense stink and heat".