"20,000-fathoms-deep world"

An apparent reference to Jules Verne’s popular 1870 science-fiction adventure, Twenty Thousand Leagues under the Sea. The unit of measurement is wrong: a fathom is two yards (or six feet), and a league is roughly three miles. Hence, Verne’s novel is about a submarine that travels 60,000 miles around the globe beneath the surface of the ocean; not straight down, as “20,000-fathoms-deep” would imply. Bradbury might have had fathoms on his mind because his famous short story, “The Foghorn,” provided the inspiration for a Hollywood movie called “The Beast From 20,000 Fathoms” in 1953, but that would have been after this novel seems to take place.