Once you have created a deep mine shaft and placed it in into the 3D World you can go deep below the surface
and start gathering rare metal ore and other things...
If you step in front of your deep mine shaft you can enter it by using the "interact"-key (default: return).
Now you are deep below the surface in your mine.
In the actual version of the game you can not dig deeper than level 1 and you dont need to support carved out areas.
These features will come within the next game update.
As you can see you have already carved out a small place around your ladder to the surface.
To gather resources you must first crush a wall (shown here as circles with the letter “W”).
As soon as you have crushed the wall the tile behind it will fill with rocks (R) that you can crush to gather resources.
If you want to return to the surface, just step near your ladder and press the “interact”-key