River Styx
From Tardis Wiki, the free Doctor Who reference
The River Styx was a mythical river in Greek mythology where the deceased have to cross the river in order to enter the Elysian Fields. When Katarina (who was from Ancient Greece) died, she ended up in the afterlife, and had to cross the river Styx to get to the fields. (PROSE: Katarina in the Underworld)
