Skills for Underground Construction

[Prince Charon]

From dugout shelters, to WWI trench lines and Viet Cong tunnel networks, to the mansion-like Great Smials of Tolkien's Took family, holes in the ground (or the sides of hills) to live or work in are things that may be important for PCs and NPCs in role-playing games. How are they made (game-mechanically), though?

There does not appear to be a 'Digging' skill in GUPS, which on the one hand makes sense as it's 'unskilled labour,' but on the other hand, it seems to me like some sort of HT-based 'digging' skill would make sense for much the same reasons that Hiking and Running are HT-based. Basic Entrenching is probably covered by Soldier, but you probably need something like Professional Skill: Miner to know where and how to shore up tunnels so that they don't collapse on you (and indeed, miners have created some pretty elaborate places deep below the grass). The trench lines of late-WWI were quite elaborate, with shelves and bed-nooks dug into the sides of trenches, and even large subsurface rooms for storage, meetings, kitchens, and officers' quarters. When the guns weren't too loud, soldiers might have imagined themselves as being part of some strange community who simply chose to live underground (it is very much to Tolkien's credit that he was able to make something so beautiful out of the trauma the war gave him).

Designing underground dwellings probably involves something like Architecture (Underground), though there may be a better name for the specialization, and for magical folk who live under the hill, perhaps some form of Symbol Drawing (Sacred Architecture). Artist (Sculpting) is likely involved in decoration, but might also be used to make stone furniture.

What other skills, techniques, or similar might be involved? How might this work, if for example the PCs were members of a halfling or dwarf community settling in a new location?