In Greek mythology, the river that souls must cross to enter the Underworld is called the Styx. This river serves as a boundary between the world of the living and the realm of the dead. Souls are transported across the Styx by Charon, the ferryman, who demands payment for the journey, often a coin placed in the deceased's mouth. This river symbolizes the crossing into the afterlife, emphasizing the finality of death and the transition to an existence beyond mortal life, an important theme in Greek mythology concerning fate and the afterlife.