I'm attempting to store vertex data in a quadtree with C++, such that far-away vertices can be combined to simplify the object and speed up rendering.

This works well with a reasonably flat mesh, but what about terrain with overhangs or tunnels? How should I represent such a mesh in a quadtree?

After the initial generation, each mesh is roughly 130,000 polygons and about 300 of these meshes are lined up to create the surface of a planetary body. A fully generated planet is upwards of 10,000,000 polygons before applying any culling to the individual meshes. Therefore, this second optimization is vital for the project.

The rest of my confusion focuses around my inexperience with vertex data: How do I properly loop through the vertex data to group them into specific quads? How do I conclude from vertex data what a quad's maximum size should be? How many quads should the quadtree include?