The Ergen Empire is thought to be the largest in the history of the Four Corners of Civilization.
Originally composed of hundreds of cities, all but eight were destroyed during the onslaught of the Creation War. They are named within the text as, Belen, Antus, Vaeret, Tinusa, Emlen, the twin cities of Murilla and Murella and Myr Tariniel.
In The Chronicle Edit
The Ergen Empire is described during the story of Lanre, narrated by Skarpi in Tarbean. It addresses Lanre's death and subsequent revival by Lyra, as well as the partial history of how Lanre became Haliax. The story also dictates how The Chandrian betrayed seven of the eight standing cities in the empire, thus destroying them. Upon the destruction of Selitos' beloved Myr Tariniel, they are cursed and as such a single city is saved.
The University and Imre are located in the Belenay-Barren, indicating that the city of Belen may have once been near that part of the central Commonwealth. It's possible, then, that the Underthing may be the remnants of the ancient city.