The S.O.L. Units in Unterheath are large pieces of machinery that are mounted on the roof of cave in which the city resides. These units are massive lighting fixtures that move across tracks on the ceiling of the cave and provide light for the citizens of Unterheath. Due to being stuck underground, the idea of light is very precious to the citizens of Unterheath, and that is why such a religious following has grown around the S.O.L. units.