Below the zero level is defined as a surface that is assigned to the zero level.
On Earth, the zero level for reference heights are often defined on the basis of a sea. Depending on the height definition, this area corresponds to a rotational ellipsoid, geoid, Quasigeoid or other irregular surface.
For planets with solid surface as the zero level applies the surface of the equal-volume ellipsoid of revolution.
On planets without solid surface, it is the level at which the gas pressure is 1 bar.