Is inverted terrain possible?

Hello,
I am currently making a mining game. I have the player spawn underground and they are able to mine terrain. This goes infinitely in all directions, and the way I am achieving this currently is by spawning the terrain in chunks around a chunk that has been mined for the first time. This works pretty well, however it would be better if I could invert the terrain.

The way that this would work is that, because terrain works voxel wise (0 being empty and 4 being full), I could set the default voxel to 4 instead of 0. I could be severely misunderstanding terrain here. Do voxels only spawn where there is terrain?

The other way would be to simply invert the faces of the terrain. If this is possible, the player won’t be mining terrain but will be creating it. However, since the faces would be facing inward, it would appear as if they were mining it. Is it possible for this to be the case, or should I use my chunk method? This is the best case scenario.

My last idea is to create a sphere of terrain where they mined, and then carve it out, so it essentially extends a shell of terrain around. The problem with this is that I have no idea how to detect where they have already mined to prevent the shell from spawning there.

I’m sorry if this is hard to understand. Thank you for reading this far!