The underworld was a concept of the afterlife in several religions both on and off Earth.
In Mesopotamian mythology, the underworld was ruled by a queen named Erishkigal. (PROSE: Timewyrm: Genesys) Ishtar was known for descending to and returning from the underworld. (PROSE: Walking to Babylon)
In Greek mythology, Hades was king of the underworld and Charon was the ferryman who carried people across the River Styx to the underworld. Orpheus was known for going into the underworld to get his wife back from the dead. (PROSE: Deadly Reunion) Katarina believed that the Doctor's TARDIS travelled through the underworld. (TV: The Daleks' Master Plan)
In Pompeii, the Romans believed that the Pyroviles were the gods of the underworld. (TV: The Fires of Pompeii)
In Quill mythology, the Quill Goddess emerged from the underworld, where she had been trapped, into the first Quill nest. (TV: The Metaphysical Engine, or What Quill Did)
Maximilian Vilmius called himself the king of the underworld. (AUDIO: The Ghosts of N-Space)
The Fourth Doctor called the planet which had formed around the P7E the underworld. (TV: Underworld)
Seb called the Nethersphere the underworld. (TV: Dark Water)