Paul Ernst's "The World Behind the Moon" (short story, adventure, free): Did you know earth has a second, invisible, moon too?
Yes - it's called Zeud. It's about the size of moon, & moves behind moon, always staying invisible from earth! And its teeming with life - monsters, sentient tribals, jungle, ...
Super-inventor Professor Dorn Wichter & a young man named Joyce go on adventure here, traveling in professor's "water-motor" driven rocket, after they notice Zeud during their initial adventure on the far side of the moon.
Fact sheet.First published: Astounding, April 1931.
Download full text as part of the magazine it originally appeared in from Project Gutenberg or Manybooks.
Among the stories from Astounding/Analog issues edited by Harry Bates.
Related: Fiction from 1930s.