Any section that's left without a direct connection to the ground surface will detonate after mining, and this applies if it's all above or below the ground. There are techniques for destroying entire ore stones by completely burying them first, you might have seen the occasional silver ore do this naturally.

Originally posted by mrjepson:

I have undermined large rocks in both the mountains and black forest, then broke them entirely by breaking a single piece similar to the ones in the plains. If multiple rocks overlap each other you need them to be not touching for it to work.

How do I do this though? I've successfully mined under only one copper vein in all my playtime and now the terrain just arbitrarily doesn't let me get low enough to mine the whole support away. Would also really love it if the terrain didn't fill ore anymore like it was hollow.

I have undermined large rocks in both the mountains and black forest, then broke them entirely by breaking a single piece similar to the ones in the plains. If multiple rocks overlap each other you need them to be not touching for it to work.

the smallest, hair thin bit can support tons. And, like tearing down a building, it can take an oddly long while sometimes to decide to fall even after its all cut. Small rocks will just fall to the new terrain level without breaking. So, yes, all 'larger' rocks that can be broken (a few can't!) will shatter if you make them fall.

It depends. If you are talking about rock build pieces, you can get about one half of one block hanging in the air. Anything you add after that will crumble.

If you're dealing with natural rock or metal deposits, you can undermine them until they are entirely hanging in the air - no support whatsoever, and just leave them like that for the rest of the game. They won't go anywhere.

The bounding boxes on all rocks are really clunky, so don't trust that you'll break them by digging under them. You can actually build a floating base pretty easily if you get semi-lucky, but I haven't tried much beyond a few structures on them.