The Underworld is for immortals only. If a mortal ever travels to the Underworld, they must pay a price to be released. Eurydice was one of the mortals that secretly went to the Underworld, and she got killed by a snake there, as she was prancing and dancing in the valley of snakes.
The Underworld is surrounded by a series of 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 swear) named after the goddess of hate, Styx. and The Lethe (river of forgetfulness).
Persephone and Hades are the king and queen of the Underworld. Hades abducted Persephone and made her the Underworld's queen.