Thanks
I don’t know how many kind of terrain Panda3D have, but holes will be simple, delete some polygons which will be covered by models as entrance to underground. Underground will be modeled by artist in 3d modeling program.
I don’t know what technique is used for this (in Panda3D) and how many they are. I only know two, one where some terrain chunk is deleted and covered with model as entrance to underground. Two is voxel terrain.