Excerpt from J.U. Lloyd's novel Etidorpha -

(upon descending into the earth)

..their weight decreases steadily as they descend, and soon they are able to leap into the air and float forward for great distances, covering a dozen miles in a few minutes. I-Am-The-Man discovers that the deeper he travels the less frequently he needs to breathe and eat and the less often his heart beats. Finally he ceases to breathe and eat altogether and heart stops beating, yet he feels more alive than he ever has; the guide explains that with little gravity and a nutrient-rich atmosphere, the inner world is much kinder to the human body than the outer. A man living in the inner world can live for centuries and hardly age a day, for there is no strain on his body at all.