At one stage or another perhaps there was a mix-up in my brain about what parts of the hive are composed of what, but I imagine the Underhive to be a varied terrain of sludge rivers and seas, hive ash (a catch all term for fine grained garbage, slag and other material that has over time been ground to ash or sand like consistancy), piles of junk and hive structural components. It's just all the rubbish that filters down from the central and upper hive (run by the Houses), which are mostly industrial in nature, and as such produce a lot of waste. This could cause some confusion as the terrain outside the hives on Necromunda is referred to as ash wastes.
Beneath this wasteland that is the Underhive surface we have the domes - much older structures again, long abandoned and only inhabited by outlaws, mutants and beasts. These are represented by the caverns in DF. We would have liked to put a sump sea at the bottom (instead of lava/HFS) but unfortunately this is not possible within the confines of raw modding.
If anyone here knows their way around interactions, I have a problem open
here that I have not yet been able to resolve.