* ScifiWritersHaveNoSenseOfScale: The survival of the human race and of evolved predator races is based on energy and light provided by volcanism on the bottom of the Great Valley. It's explicitly said the upper surface of the Earth is frozen cold (and supposedly just as inhospitable as a cold planet like Mars today). However, the Great Valley is said to be more than 100 miles deep where the Great Redoubt lays and maybe 150 miles where the Lesser Redoubt stood. The highest mountain today is 5.5 miles high and everything above 5 miles is regarded as the death zone, where nobody can acclimatize, pressurized aircraft fly 8-9 miles above the Earth and the limit of space is 60 miles above us. Yet our hero ''walked'' in armor outside the controlled atmosphere of the Redoubt and descended ''50 miles'' through the Valley? And nothing changed around him outside "a bit thicker air"!? Saying that air on the Earth surface nowadays is a bit thicker compared to air 50 miles above is like saying the Pacific is slightly wet.
** This is possibly more of a case of ScienceMarchesOn given when the story was written; the very term 'death zone' was coined 41 years after this was written, and much of the research took place after the author's death in 1918.