The Sanzu River is a mythical river in Japanese folklore, often associated with the afterlife. It is believed to separate the world of the living from the realm of the dead. Souls must cross this river to reach the afterlife, and it is said that they are guided by a ferryman. In some traditions, the Sanzu River is depicted as a place where the deceased must face challenges, such as paying a toll or overcoming obstacles. The river is often compared to the River Styx in Greek mythology, serving a similar purpose in guiding souls to their final destination.