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Mountains - Generated about like water; impassable terrain; can build tunnels/caves using explosives
Subterranean Layer - generated with mountain, can contain ore patches and even biter nests. will allow for passing through a mountain by foot, car, or train.

https://mods.factorio.com/mod/Decu

A scenario where you can go underground and dig through rocks. You could find treasure chests, patches of ores... or big caves full of bitters! Credit to original creator Decu

yes, but this is a scenario, so it's not even really a mod.

So ?