Eros was a roughly spindle-shaped rock only six and a half kilometers thick at its narrowest point. Since the surface of the planetoid was entirely devoted to absorbing sunlight and converting it to energy, the asteroid's inhabitants lived in the smooth-walled rooms linked by tunnels that laced the interior of the rock.
During the Second Formic War, the Formics carved great tunnels into the asteroid's interior, and used it as a base. The Formics were driven out, and the asteroid later became the base for International Fleet Command and Command School.
- In Ender's Game, it was stated that humanity's first encounter with the Formics was on Eros when they slaughtered a crew sent to investigate the asteroid. However, this was contradicted by Earth Unaware.