Deep mining

It would be great if, in addition to open pit mining, you could also build collieries and thus deep mines with tunnels and shafts

Had the same idea. Got some additional ideas:
- Deep mining must be researched and has different tiers to get deeper and deeper.
- You need to set up a Minshaft building that is the conection point to the mine. Could maybe also have a ore sorter integrated. The mine building has outputs for the ores but also inputs for supplies.
- A new item "mining supplies" in different tiers (analog to construction materials or electronics). The basic ones could simply include wood to symbolise the support beams you need to set up the mining shafts. Later versions could need concrete, steel, electronics. Maybe also cloth (if that is implemented someday)
- Mining with the shaft will need more workers and also unity (because nobody likes to work underground) but therefore needs less gasoline
- Also you have more space to build stuff since there is no need for mining pits.
So in the end using deep mines will be optional. It should give some benefits but will also need some special supplies.

I would like deep open mining ,way deeper than now.