It orbits an old moribund star, a Red giant, on the outer edge of the galactic north. The temperature outside the sealed Hives is below zero, always freezing or frozen. Necromunda is too far away from its sun to receive any real warmth, and despite the clogging clouds above, the temperature is never high enough to allow life to live in comfort - if it could live at all. The chemical rivers and sludge sometimes freeze over into huge, icy mirrors which the Tech-Nomads may sometimes safely cross. The planets rocky crust is scarred by the shifting corrosive ash that is the ground down waste of eons.
Water is found only in deep wells, geothermal steam or the bodies of humanity.
No life lives there without the aid of high-tech., life support or sealed vehicles. Necromundas natural resources are exhausted. The Hives survive through constant re-cycling of wate materials. There is little left on Necromunda that is useful to the Imperium - except the slag factories that turn waste or recovered metals into ammunition and the roving gangs that make Necromunda's brutalised people born warriors for the Imperial armies. No challenge to their status is intended.