What is the name of the Hawaiian underworld?

The Hawaiian underworld is called Lua-o-Milu.

In Hawaiian religion, Lua-o-Milu is the land of the dead and is ruled by Milu. It is not simply a place of punishment: traditional accounts describe it as a distinct realm where the spirits of the deceased continue to observe the world of the living.

One important image in these traditions is the leaping place. A soul could depart from the top of a valley wall or sea cliff, sometimes reaching the next world through a tree. Hawaiian traditions identified such departure points on each island; Leina Kauhane is another recorded example, while Waipio Valley was associated with an entrance that later became hidden by sand.

The answer is sometimes confused with Po, a broad Polynesian term associated with darkness, night, or the spirit world. Lua-o-Milu is the more specific name for the Hawaiian land of the dead, while Milu is its ruler. Some stories also warn that spirits there could watch the living and even turn them to stone with a stare.