This hollow earth nonsense of his is almost as embarassing as his failed 1-issue comic title "Skateman" which featured the line “What the shit do you care, pig-meat?”
He has an entire series of a Batman story where Batman and Robin find a way into the middle of the Earth where there is a secret world filled with Dinosaurs and Cavemen. One of the Cavemen has decided to be the Batman of the Center of the Earth and dresses like Huntress. His "Robin" is a Human/Dinosaur hybrid named Primus.
http://www.comicsalliance.com/2012/08/29/batman-odyssey-review -neal-adams/
That is the third part of the review of the entire series, but it's the part that has the most to do with being in the middle of the Earth.
If the earth were constantly expanding, therefore, you might be able to observe a visible decrease in the levels of the ocean over time due to large amounts of liquid water no longer being transported to our planet via meteors.