What I think is, to upgrade the bunkers, you could use iron, and it would only have to be 1 block thick.
so instead of having this:
you would only have to make this:
The insides are the exact same, and it's one block underground.
Just came across a mod that referenced this thread, and is perfect for this: Solar Apocalypse. This mod makes pretty much forces you to live in a bunker or underground. I mean, sure, it's not nuclear apocalypse (although it kinda is, sun is nuclear :tongue.gif:), but it's close and would impose almost the same limitations.
There's also the Sunburn mod that's a bukkit server plugin (which does exactly that, sun burns always).
There's also a Wasteland Mod that turns all the world into dirt, trees into leafless trees, and add some ruins with chests.
I'd also suggest, if you want, use the Animal Transport mod so you can make lasso and carry animals to wherever you want to breed them :tongue.gif: (For atmospheric purpose, obviously this kinda goes against the mod's intention of making things hard)