Here's my problem with the idea of zombies digging underground : it makes underground bases impossible to defend.
Above-ground, you can surround your base with any or all the crafting system offers.
Spike pits topped with barbed-wire and electric fences, backed up by shotgun/auto turrets and sniper perches from which players can practice their marksmanship skills - you name it, it's perfectly viable.
Underground, none of these things work unless you hollow out a massive cavern within which to build a standard surface-style base.
I don't know about the other mole people, but that's not how I build when I go underground. I carve out my living/working spaces and line the floors/walls/ceilings with whatever material strikes my fancy.
If "digging zombies" becomes a thing, they will be a thing we can't do anything about until they're actually inside our bases - unless we completely redesign our dwellings to create that open space that's necessary to implement proper defenses, at which point we might as well build on the surface because underground dwelling will have become objectively worse than staying above.
As it stands, underground bases provide a reward for effort that's similar to farming. It takes a lot of time & work to create one, but it's worth doing because it makes you safe. You sacrifice the thrills and rewards of fighting for your continued existence every Horde Night in exchange for the "snoozefest" of simply enjoying that continued existence without having to fight for it. Stilt bases provide the exact same benefit, provided you build high enough to be out of vomit range.
If you think half of the world is effectively closed-off to you now because it's "too safe" or "too boring" to live there - you have the option of simply not going there. What's going to happen if that part of the world is made equally (arguably
more) dangerous, however, is that a lot of players will have had their "safe zone" forcibly taken away from them.
Should building underground have additional complications? Absolutely! I just don't want to see the option become
less appealing/viable than the alternative.
I rather like the idea of ventilation requirements, and completely agree they should be universally applicable rather than only being enforced underground. It'll take more work to ventilate deep dwellings, just as it takes more work to create underground farms, so the system shouldn't require any artificial balancing measures.
The idea of underground radiation which can be negated by protective measures is problematic in my opinion because
- Hazmat gear cannot be crafted, so your ability to safely go underground will be directly linked to RNG
- Once the player has hazmat gear (or has lined the base with lead blocks, or crafted radon scrubbers, etc) the problem completely vanishes
I also like the idea of requiring periodic surface time, getting fresh air and sunshine - but as with the ventilation concept, I don't think it should be strictly an "underground vs aboveground" consideration. If you're spending all of your time completely surrounded by concrete, the elevation of your residence is immaterial.
My suggestion? Implement two new systems:
- Air - exactly as Pinz suggested, make improperly ventilated areas behave as though they were underwater.
Ventilation shafts to work in much the same way that we bring light to underground farms with perhaps a greater effective radius, which can be further expanded upon with powered machinery.
- Windows & doors adjacent to air blocks which have line-of-sight to the sky can be treated as air blocks, meaning surface dwellings with good window-to-area ratios won't have any problems at all.
- Lastly, to make it possible for players to improve on their ability to function underwater and in poorly-ventilated areas, give us a breathing skill which functions like survivor and camel.
[*]Sunlight - to be handled in fashion similar to hunger & thirst.
- Extended stays underground or cooped up indoors will drain this meter, eventually causing an erosion of your wellness.
- Replenish the meter by spending time outdoors when the sun is shining.
- Meter neither drains nor replenishes while outdoors at night or during bad weather.
Feel the need to contend with zombies, despite your base being 40 meters underground? I have no problem with their AI being tweaked to make them much more interested in doors, hatches, and tunnels. Force me to trap/defend my entrances in order to keep the undead out of my home sweet home, but don't let them dig through native rock to get to me.