I find the hollow earth theory fascinating. Very interesting to consider the Inuit and Taymyr legends.

If I recall correctly, at one point drilling one of those super deep holes in Russia, they struck salt water? The ocean on the inside? Very interesting.

Also in certain NASA experiments (so take with a grain of salt), they were able to demonstrate that in zero g, spinning bodies of water will form voids in the center, perhaps due to centrifugal force. Perhaps the same happens with planetary bodies?