I think they either fall under industrial if you don't get your ass up them or High Stuff if you do.
Trouble will sub-forums is where to stop? Culvert, drain, railway tunnel, air raid shelter, mine, cave, underground reservoir, cellar, basement, road tunnel, vents, service tunnels, storage tanks, wells and I'm sure there are many more dark places without including all the military stuff.
Leave gas wherever people post them really. Some are awesome just looking from below like West Ham, North Thames & loads of other small ornate ones, & some are better from up top, so the format will likely change between them. Plus they're almost all named incorrectly compared to gas companies current records, so there's still a minor level of hiding there, albeit very low level, even in public. Put them in a sub-forum together & as soon as a gas company finds that, there'll be a load more ladders disappearing so it will be counterproductive for anyone wanting to access more.
I may be missing something? But I really liked ( on the old site ) how on a members profile page all their actual reports could be found easily without having to scroll through all their posts first, I found this search tool good when a user i'm not familiar with asked about entry's and locations, ( no reports up = no access tips )