Chapter 75: 733-743
Page 739
"[T]he Earth's Surface [...] began to curve sharply inward, leaving a great circum-polar Emptiness [...] toward which our path was taking us [...] And 'twas so that we enter'd, by its great northern Portal, upon the inner Surface of the Earth."
Thus, Dixon relates to Mason his journey into Hollow Earth, guided by what seems to be an extraterrestrial being. See p.548.