Hollow Earth

Pretty well spelled out in the film. Here's a link to an answer that has a couple of images directly from the film showing what and where. https://www.quora.com/Can-someone-draw-me-a-cross-section-map-of-Earth-including-hollow-earth-according-to-Godzilla-Vs-Kong/answer/Stephen-Breen-26
How... is another question and essentially is answerable by the words: This is a Kaiju Fantasy, and the second word is really redundant.