Opeton City, also referred to as the Underground City, is a long-abandoned and partially ruined subterranean city on Eamon that is the setting of Steve Adelson's adventure The Underground City. Its location is described as being beneath the plains near the Main Hall in Evenhold. Its name is given on a sign in one of the receiving stations.
Physically the city consists of many interconnected white buildings occupying a vast, stone-ceiling cavern, with multiple "elevator tubes" leading up through it to the surface. The city's builders are unknown but were extremely technologically advanced and capable of building robots, levitating structures, and even time travel machines.