The Hollow_Earth versus deep underground pockets or caverns, are different things.
https://www.vice.com/en_ca/article/9bne3p/this-lady-thinks-the-centre-of-the-earth-is-hollow-and-full-of-alien-humanoids
For the earth to have an enormous empty center or void would mean that its quite a different planetary structure.
http://homepages.uc.edu/~harmonti/hollow/index.html
There could be vast underground pockets large enough to contain ancient and modern cities but the public would never be told of such things.
http://www.foxnews.com/travel/2013/04/09/8-amazing-caverns-in-us.html
https://en.wikipedia.org/wiki/List_of_longest_caves
