The theory refers to the Geocavitic sphere theory, or thT as the earth was firming in its.molton state the planetary spin created a Hollow or concave within the center similar to the Hollow created by a centrifugal force of a horizontal washing machine flowing a spin cycle.
The theory states that the shell of the earth averages between 800-1000 miles thick with an interior surface consisting of oceans and land Illuminated perpetually by a sphere of electromagnetic and or nuclear energy suspended at the very centre if the empty space.
There are reputedly fuel like openings near the polar regions perpetually concealed by a mist created by the collision of cold air from the outside and hit air from the inside which permits the ingress and the egress to and from this inner world.
Rhe theory states that the inner surface has his own gravity, yet slightly less that the outside gravity.
One side theory is that between the inner and the outer surface where the gravity is nullified there exists a layer of weightless or low weight caverns 8n an eternal state of chaos where minerals liquids, gases and chemicals continually slams together from the earth rotation causing intense magnetic activity, a virtual inferno a bottomless pit.
Bibliography
Underground Alien Bases Flying Saucers Co.e from Inside The Earth! by Commander X