The Underworld is where you go to die if you happen to be named Mario. If you are not, you’re reincarnated as a blade of grass. The land is ruled by Hades and his son Zagreus also lives there. The game is the main setting of the game Supper Mario Slain: I Died Once where Mario goes to because he name is Mario.
The Underworld is completely dark and is impossible to see anything in. The terrain itself is rocky.