The 'City of Um' is a legendary city referenced in the prophecies of ancient Xolhengianism, which is believed to be a Structian city sprawling out across the Impendian basin, now covered by dense hydrocarbon smog. Scientists are fairly certain no structi can exist under the atmospheric pressures that exist deep in the basin where the city is thought to be, and no evidence of the city has been found in expeditions or with sonographic imaging.

In Xolhengianism, the devout believe that an incarnation of the god Um will slay the demon of the red star Rut and his smog creatures, revealing the city where Impendians will spend the rest of eternity.