The core, Richard Shaver & L. Ron Hubbard.
How much do we really know about the core? What is at the epicenter? What powers it? What's the difference between a dead world and a world with all sorts of tectonic activity? The lack of progress in this area makes it still a ripe ground for sci-fi.
All those hollow earth folks are remnants/descendants of proponents of The Shaver Mystery. You're old enough to have heard about that, right?
It dates to the 1940's when sci-fi author Richard Shaver described how he stumbled upon an opening/entrance to a hollow earth. Sci-fi folks a bit around the bend went hunting for Lemurians and openings to this inner earth.
Pretty sad actually. Very similar to L. Ron Hubbard abandoning science fiction to cash in and create a religious cult.