If a room is DARK, then when people besides the owner 'look' there, they only see things they own. If a thing or player is DARK, then "look" does not list that object in the room's Contents:. Players can set rooms they own dark, and may also set objects they own dark, but they cannot drop dark objects in rooms they don't control. Wizards can set players dark. When a program is set DARK, it is called DEBUG, and a running stack trace is printed out to any users of the program for each instruction executes.