Cater makes some nice points about the creation of caverns way, way deep within the shell of the Earth.
"The sliding of plates over each other no doubt produced the large caverns which honeycomb the Earth's shell. It should also be noted that if the Newtonian concept of gravity were valid, the enormous pressures at such depths would produce cold welds resulting in a totally solid Earth with no plates."
Cater is saying that UNDERNEATH the continental plates, there are huge caverns, as large as entire regions.
Remember, Cater stipulates an electromagnetic cause to gravity. He makes the point in the same chapter that the gravity-producing radiation doesn't penetrate the shell completely because it changes frequency during its passage. [How far do radio waves penetrate a wall? Not forever and ever] And also that the accumulation of particles within a given cavern will block the gravity-radiation from penetrating it. In this way, there is no danger of superior weight collapsing the cavern world.
Related thread: Gravity Shielding, in the Depths Below — Continues the June 2017 Cater gravity/cavern argument