The Underworld

According to the ancient Greeks, the kingdom of the underworld was where the souls of the dead resided.
The kingdom was usually located underground and to get there souls had to cross the river Acheron on a boat ferried by the ancient Charon.
To pay for the journey, an obolus, a coin of little value from ancient Greece, was placed in the mouths of the dead.
The term is still used today to refer to small donations of money.