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.