Pneumagination chunks are now completely hollow except for the outer layer and chunk borders. These brushes become visible as you break through the ground.

Since brushes can have any vertex moved there’s no way to do any simple open-air detection for visibility. This however does apply to the simple cubes of the main terrain as long as no neighboring brushes have been moved. I’m fine tracking extra data in each brush if it means removing like 80% of the mesh.