"The Horror of the Heights" (1913) was a story by Arthur Conan Doyle where a bold aviator in the early days of flight becomes curious about why pilots seem to die between 30,000 and 40,000 feet. When he goes up there himself, he discovers the reason: around a high enough altitude, there are "air jungles," entire unearthly ecosystems of lighter than air and semisolid life arranged in eerie and otherworldly structures...including the equivalent of predators, gigantic tentacled jellyfish creatures.
The story was written as the diary of an aviator discovered in a crash, and the final sentence of it was chilling:
"Forty-three thousand feet. I shall never see earth again. They are beneath me, three of them. God help me; it is a dreadful death to die!"
Though best known for creating Sherlock Holmes, Arthur Conan Doyle wrote a few horror-scifi stories in his time.
As the story was originally published in the Strand magazine, the biggest literary magazine in the world, it was hugely influential. For a long time, even into the 1950s, there was an idea that the explanation for UFOs was that they were not vehicles or even extraterrestrial at all, but some type of animal, possibly of squid-like or jellyfish like intelligence and behavior, able to live in the upper atmosphere without touching the ground. As Charles Fort said in 1913, "it seems no more incredible that up in the seemingly unoccupied sky there should be hosts of living things than that the seeming blank of the ocean should swarm with life." The existence of aerial upper atmosphere life forms is something that was baked into a lot of early 20th and late 19th Century culture, like the famous Crawfordville Monster of 1891:
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