Origin: Chinese with other influences
The Underworld, sometimes known as the Grey, the Other, the In-Between... In the Ohlone cultures (the natives to the area), the land of the dead and the in-between is called Chochenyo. In the regional Chinese cultural beliefs, it is referred to as the Diyu.
Similar to purgatory, it represents the place spirits roam who are not ready to move on. In the Diyu, the real nature of people, places and events is visible.