In chapter 17 of THE BEST OF THE HOLLOW HASSLE (a newsletter from the 1980's), we read about how our planet may have formed as a hollow or geoconcavitic sphere... as in a more molten state it spun and pushed most of the matter out from the center, like the end of a laundry machine cycle, leaving a 'hollow' space within.

"...The densest portion, the crust, has two natural explanations. Untold eons of spin have thrown out the heavier matter toward the periphery (we might imagine, then, that the finest, most subtle matter remained at the core). Considering also the accumulation of cosmic dust, the debris settled upon our surface through the march of time/space, the earth has thickened its "skin". The theory of continual drift falls naturally in relation to that of an expanding planet, simply busting out of its britches. It seems quite likely that a planet, like other living things, grows. Certainly Geology, Archeology, Paleontology, etc. know all this. The stuff underneath is old, underneath it, older; the further you go the greater the antiquity."