Perhaps like geodes?
Did the Earth expand or is the Earth still growing and if it is could that mean an internal hollow space(s)?
Would it be easier for the planet to expand if the Earth was originally hollow?
There were/are mentions on many websites of large openings, especially at the polar regions, that are said to lead into the Hollow Earth or a hollow layer beneath the surface. Are these ideas now in trouble due to satellite technology or are they literally being covered up in things like Google Earth?
Hollow Earth theories
Edmond Halley, the discover of Halley's comet, in the 17th century put forward the theory of a Hollow Earth. His Hollow Earth theory consisted of multiple layers.
Is the Tamarack Mines Mystery evidence of a hollow Earth or that our planets internal structure may have its own mysteries?
Concave Hollow Earth theory - a bit of a visual and mind bender, the link to wikipedia explains this hypothesis in detail.