Written in 1932, this is a classic mash-up of a Mars part-derived from Edgar Rice Boroughs, the Lovecraft of Mountains of Madness and the period lore of ancient civilisations (like those being explicated in the popular press of the 1930s) pushed back 40,000 years and across the solar system. The bottom line is leech-like brain eaters lurking in the bowels of the planet and you can see the link between 1930s pulp and the C movie horror films of the 1950s here but there is genuine awe and terror in this story and it is justly regarded as one of the fantasists' best. This story was first published in Weird Tales, May 1932.