Earth is not hollow and Mars is far away., but did you know the moon have gigantic underground dungeons? One is located underground in the Shackelton Crater, likely 1km tall and wide which can easily fit the worlds largest city. And that's only underground, because they could build an atmospheric dome over the crater. Why are we focusing on Mars before we even try living at moon? The moons gravity would make us live to 160 years old they say. You could even log onto internet and game if you want because we are close enough to earth.

http://time.com/4990676/moon-cave-base-lunar-colony-exploration/
https://www.youtube.com/watch?v=BAzHsMY0ZJ8