system
Given the following post, respond as if you're commenting on Reddit:
human
[Necromunda] What prevents 10-mile-high Hive Cities from collapsing on themselves?
gpt
Rockrete and Adamantium: super materials that we haven't and may never invent.

Also remember that there are vast sections of the Underhive that have collapsed due to the weight above them, forming denser and (likely) more supportive sections underneath the new construction.  It'd be like having an arched roof that can hold a certain amount of weight, then having those materials collapse into a pile of rubble.  As long as the rubble was stable enough, you could build a much heavier structure on top of it.