The Coming Race, written in 1871 by Edward Bulwer-Lytton, was one of the first lost world novels and a prototype science-fiction novel.
With the known polar diameter of the earth, and its difference from the equatorial diameter, we can calculate (with a few assumptions) the size of the openings. Allow me to demonstrate:
Unlike, the flat-Earthers, the hollow-Earthers believe that our Earth is spherical but with a hollow body. Their conspiracy theories also suggest that the moon, the stars, the Sun, and other planets are all hollow bodies.