Bigger open mine

We need bigger and deeper open mines.
Now i don't really need the retaining walls.
But if someone would need to go really deep then you have to plan or adapt. So please put the resources floor deeper waaaay deeper or some resources not at ground level but way deeper before you can reach it.

Maybe there are horizontal veins buried underground, and they can be discovered (don't normally turn up in Layer view) by mining in to them, or some sort of special ground scanner? Could be random unlike the other deposits, even under water.