Bring back caving

[cdfa]

Context

I haven't played through the full pack yet, but it seems that there is no reason to go caving at all, except for in the betweenlands where there is normal ore generation. I think this is kinda sad because caving provides a nice break from the continuous following the progression tree and is a nice challenge, especially early game.
I also like the "finding samples and digging down" approach, but it's a lot less exciting, so I would suggest having some ores generate more like in vanilla, or creating another reason to go caving.