The theories of a "hollow" earth are indeed fascinating. The most prominent theory is probably the "Polar Holes" theory. One of the strangest models is the "Inverted Earth" theory ... truly bizarre. A more recent model is called the "Complete Shell" theory and it accounts for modern observations of gravity which contradict assumptions based on Plate Tectonics. One thing can said for sure, current theories on earth's formation are half-baked, i.e. wrong.