It's a LOD thing, isn't it?
So if I want to make sunken park example, i do it normally and then drop below Z axis. Then just agjust LODs too underground or what? Tutorial would be nice :)
You only need underground LODs if you plan traffic pass trough the model. If this is not the case, a simple plane at z=0 will be a good LOD for an underground model.
Thank you :)