First of all, I’d like to post the 2017 GDC talk Innes McKendrick did on Continuous World Generation. After reading your explanation, I believe this is for a great deal actually based on that talk and it would be good to have the resource available. It has most likely been posted elsewhere already, but Discourse has yet to implement a way to index such embeds.
Now to give me my thoughts on actual mining by going down into the terrain. This is certainly possible, within of course the limits you mentioned, which is the 128m height range HG decided on, resulting in acceptable distortion between sphere calculations projected on a cube. The terrain is broken down into 1 meter voxels, resulting in a 6 byte voxel size to store density, 2 material types, and blend information. So basically adding additional terrain that can be mined, whether existing or new types, is certainly possible, within the range mentioned above.
However, the rendering described, is mostly surface rendering, while we are discussing resources hidden below the surface. This extra data needs to either become available and be rendered once you reach it, or pre-rendered ahead of time to already be available even though not visible yet. At least some data will have to be available for such a terrain resource to exist in the first place. I assume this can be done similar to dungeons or caves or the other terrain minerals we already have, like Emeril, Gold, and alike.
Speaking of dungeons and caves brings me to another type of resource. We can of course consider a resource to be similar to terrain, but it would also be possible to have mineable resources as actual objects. This would require a somewhat different approach of course, but is very well possible too. Most of the resources we find on planets, I consider to be actual objects. You fire your laser at them and they start giving resources until they disappear once emptied. Then of course there can be objects that require interaction to be added to your inventory, like the vortex cubes or albumen pearls. I certainly think it would be possible to actually have any of the objects I mentioned, ‘buried’ inside the terrain. The same generation limit is of course still in place as to how deep they can actually be buried.
Either way, I think performance impact does not have to be big, although there are of course always limits in the amount that can be (pre-)rendered. The techniques required are already being used, so I certainly think it would be possible to accomplish mining as discussed here. It may just require some shifting around in numbers of other existing things, if render limits are already reached. Maybe decrease the number of creatures or other resources a bit to make place for additional terrain/object mining.
As for falling through the terrain when getting out of your vehicle … I have yet to experience this myself. This is usually caused by the terrain not being fully rendered, leaving a hole. Likely caused by performance limitations reached. In all honesty, I do mostly see this happen on console or low spec PCs, no offence meant. You are however correct that mining we talk about, will likely cause some additional issues. Falling through the map has certainly happened to me, mostly in places where I have either tried to mine, or use my my terrain manipulator. There are still some bugs around when it comes to properly filling up polygons when terrain has been removed. I have noticed this to often happen on boundaries between different terrain types. Of course the terrain manipulator can have some weird results anyway and can use some improvements next to it.
@toddumptious I certainly agree with wanting more variety in dungeons/caves. The resources and looks are pretty much always the same for every single planet. Although mining into the ground, looking for hidden resources is interesting, I’d rather see them work out caves some more first, or maybe a combination of both. Making further use of the ‘ceiling’ for actual resources to gather would be welcome too. In general, I wish caves to get some loving, more variety of resources, looks, ands creatures.