I remember reading this story when I was in middle school and can't seem the find what the series was called. The main character digs a hole and finds an underground city, from which he is banished an travels deeper and deeper into the earth. Eventually, the get to the center of the earth, which is hollow and has a star in the middle and find an alien race (who are trying to reproduce by laying eggs inside humans?) Not 100% sure about that last part but I think that was what happened. Also I think they said the earth was like a ships or something for the aliens, but I don't remember too well.