In the depths of the Hive City it is common for power or water to fail or access tunnels to collapse, creating unproductive toxic wastezones. The lower the region the worse is its air, power and access and the more unstable its structure. As the hive deepens normal habitation becomes impossible, and this region is known as the Underhive. There is no formal barrier between Hive City and Underhive because the border is constantly changing. Even as areas of Hive City are abandoned parts of Underhive are resettled and rebuilt. As a consequence Underhive is an everchanging frontier where people are constantly seeking new opportunities or fleeing from sudden catastrophe.
Underhive is a frontier in more ways than one. Not only is it a barrier between Hive City and the unimaginable horrors of the Hive Bottom, it is also a region outside the formal law and order of the hive. The people of Hive City live carefully regulated lives. They are protected by the strict social codes of House and Hive, dominated by family patriarchs, and obliged to work in the guild factories. The sprawling Underhive is lawless and anarchic, and its stockaded settlements form the only havens of relative order. Even in these refuges, murder and violence are everyday facts of life. Gun law is the common law of Underhive, and self-protection is the best and only reliable defense.
The physical structure of Underhive is the same as the rest of the hive. The hive probably began as a single hab-dome of plascrete or some other highly durable material. Over the years, more domes were added together with broad tunnels and shafts to connect them. Eventually, new domes were built on top of the old, and further domes were raised on top of these, producing a honeycomb of large enclosed spaces. The thick walls and foundations of these man-made caverns give the hive its structural integrity.
A dome provides a broad open space which is further divided into zones of factories, houses, commercial buildings and a myriad other structures. Each dome sits upon a deep foundation layer which incorporates a sub-level maze of power lines and supply pipes. Larger domes also have tall reinforcing pillars which support the roof. The geography of the hive is therefore one of a series of interconnected domes or caverns linked by major tunnelways or shafts, and divided internally into built up areas.
At the base of the hive buildings become so structurally dangerous that the region takes on a different and even more inhospitable character. This is the final and deepest zone called the Hive Bottom. Hive Bottom is so decayed and crumbling that the original domes and foundation piles have long since collapsed, forming a layer of almost solid rubble. Within the rubble are enclosed pockets linked by holes and tunnels worn by liquids leaking from above. These pollutants and effluents, the discharge fluid of the entire hive, form a vast lake of radioactive putridity called the Sump.
Nothing can live in the Hive Bottom other than the most monstrous mutants. Its denizens are the spawn of darkness and pollution. Some of these foul creatures find their way into the Underhive, or even into the lower parts of Hive City, but their natural domain is the darkness of the Hive Bottom.