Very efficient/fast marching cube implementation
This seems to be eluding to a Marching Cubes LOD algorithm such as: Place the entire volume in one giant cube. Break that volume into NxNxN cubes. And continue doing so until the cubes are at the finest granularity needed for the density function about 8 level works. Each volume then responds with either: No voxels - stop processing that sub volume OR Yes there are voxels -- process the sub volumes and keeping doing this until:  the voxel level is reached. Finally set a LOD - such as equating a screen space distance range to each subvolume level and treat the subvolume for that LOD as the final voxel. Cracks are a major issue with this algorithm and why you won't see many implementation of it.

There is also Eric Lengyel's Transvoxel algorithm: https://transvoxel.org

and here is a link an implementation of it
https://github.com/stoyannk/voxels