The Lost Cavern and Other Stories of the Fantastic (Vanguard Press, 1948) by H.F. Heard (a.k.a. Gerald Heard) is a collection of four short stories: "The Lost Cavern", "The Cup", "The Thaw Plan", and "The Chapel of Ease". (Synopses with spoilers follow...)
My initial interest was in "The Thaw Plan" because it briefly mentions a tree octopus, but the story's setting and nascent-Cold-War perspective are interesting in their own right: induced global warming is used by both super powers as a strategic weapon, resulting in a future Earth where humanity has split into two separate species living at either pole, separated from any interaction by an impenetrable, primeval, equatorial jungle -- home to tree octopuses, naturally.
The first part of the story sets the world-building in motion. The year is 1975, 30 years into W.W.2.A. (World War Second Armistice). The world is divided into two super powers, the US and the USSR. The seat of power in the Soviet Union has moved East, to the city of Karakorum. The Chinese have taken control, turning Moscow into a Holy City and elevating the Russian people to the highest (and most powerless) level of "Ritual Rank".
Many in the West see this as a good thing since "a Chinaman never likes war". But the new leader of the USSR, Supreme Commissar Yang, has a devious plot to diminish -- figuratively and literally -- the West (including the Russians): they will use atomic power to melt the tundra, causing sea levels to rise 100 feet, flooding most of the world but leaving the tablelands of Tibet and China uninundated, from which the USSR will rule as the lone superpower.
Meanwhile in Washington DC, US President Place -- the largest president ever! taller than Lincoln! bigger hands than Washington! fatter than Taft! -- learns of Yang's plot thanks to an observant tidal expert and spy photos showing Soviet harbors being built at the hundred-foot contour line.
He contacts and confronts Yang via a "television long-distance conference set-up" (or "T. L-d. C. S." for short, which I suppose we would uncouthly call a "teleconference", although it's described more like a janky hologram). Yang plays innocent, claiming the melting was just a miscalculation in an experiment they were undertaking for all mankind, and that they should have sympathy for the losses the USSR will suffer. With that he bows and tele-leaves.
Place, seeing that there's no way to stop the thaw now that it's begun, comes up with a plan of his own. After issuing orders and explaining the situation to the world's ambassadors, he goes on a live T.A.V. (Tel.Aud.Vis.) broadcast to tell the world what the future of mankind will be: he has already ordered planes to nuke Antarctica and Greenland, thereby melting the icecaps and raising the sea levels even higher than Yang had planned (to 250-300 feet). This will turn Antarctica into a new habitable continent, rich with unspoiled soils and untapped mineral wealth. Place claims Antarctica in the name of the free peoples of Earth as their new Promised Land.
As the sea-level slowly rises (Heard doesn't give an actual timetable, but it sounds like a multi-generational process), the East and West go their separate ways -- North and South, respectively. The Soviet people migrate to the elevations of the northern hemisphere, turning the Arctic Ocean into a new Mediterranean. There are skeptics at first in the West, and some hold-outs who try to maintain fishing communities on the midlevel floors of NY skyscrapers, but eventually the people of the West all migrate to Antarctica.
The greater level of cosmic radiation in Antarctica initially seems a blessing, creating new types of crops from the sped-up mutation rate. (Oddly, there's no mention of the radiation from the nuclear bombs dropped all over the continent -- well, this is the futuristic 1970s, so perhaps radiation-free nukes have been invented.) But the cosmic rays start affecting people, especially the cerebrotonic, making them increasingly neurotic. The only people who can deal with the neurotic effects of the radiation are the passive types, uninventive and unadventurous, who begin to dominate the Antarctic population.
(UPDATE: Apparently this first part of the story was published separately under the title "The President of the United States, Detective" in the March 1947 Ellery Queen's Mystery Magazine, where it won first prize in the short-story contest -- to some controversy since it's neither a mystery nor a detective story.)
In the centuries (millennia? -- again, no real timetable post-thaw) that follow, the flooded equatorial regions increase in temperature, spurring the formation of floating mats of vegetation. These start taking root providing footings for thick jungles to grow as carbon dioxide levels rise. New plants and animals evolve -- or rather, devolve -- as the globe-encircling jungle, now called the Central Belt -- or just The Belt -- recapitulates the Mesozoic.
The Belt becomes home to fevers, parasites, skin-eating fungi, and other nasties that make it dangerous to go there. Over time, rational prudence morphs into moral panic, and both Northerners and Southerners -- now two separate species: Homo borealis and Homo australis -- fear the place as evil and condemn those who enter it.
The second part of the story takes place long after all this has settled. It tells the saga of Ailuck, a man from the Southern fisher clan who doubts the moralists of the Temple (where President Place is worshiped as a prophet/messiah-figure) and the evilness of the Belt. His clan, who fish the waters around Antarctica, already bend the rules by visiting the Belt's outer fringes in search of a type of illicit truffle/seaweed/tuber-thing that's highly prized by epicures, including those among the priesthood. Everyone looks the other way and the hypocrisy bugs Ailuck, while his curiosity about the Belt grows.
One day while looking for the truffles, he decides to explore further into the Belt on his kayak. The further in he goes the better the truffles are. He also starts finding oysters with pearls, enough to make him wealthy. But he gets caught in a storm that destroys the canal he was using, forcing him to use another one that heads north, away from home and deeper into the jungle. He keeps going until a month or so later he comes out on the other side.
He spots land and heads for it, finding a savannah. But he gets sick from some disease he picked up in the Belt and passes out. He comes to and finds himself in the care of a H. borealis woman. She is nearly noseless, as is the evolutionary fashion of her species, noses being viewed as bestial. (H. australis are described as having hawk-like faces, similar to American Indians.) Nevertheless, Ailuck finds her quite beautiful.
They can't speak to each other, and she is never named, but through scrawled drawings and "keen mutual sympathy" he figures out that she's some sort of outcast from her people (apparently she's not noseless enough). She considers him a friend. He sticks around and helps her tend her goats for some time, until some of her people show up threatening to kill them.
Since her people would just keep hunting them if they stayed in this land, she makes up her mind to escape into the forbidden Belt. Ailuck considers her a strong, resourceful companion, so he agrees. They make their way south into the jungle, this time going through what was once Africa, until one day his companion falls into a bog and is slowly swallowed alive by a giant subaquatic leech...
Boy finds girl, boy loses girl to carnivorous annelid -- it's just one of them stories.
Anyway, he continues south alone. This is when he encounters a tree octopus (well, "land octopus", but it's in a tree):
He turned away and lunged blindly through the forest. Automatically, night by night, he coiled himself up in the lianas. Once or twice he was attacked, but the creatures were evidently so specialized that they hardly recognized this strange object as possible prey. If he did not blunder on them, they hardly ever struck at him, far less trailed or stalked him. They were sluggish beasts, whose reactions were almost automatic and who had become so specialized that these reactions were hardly ever awakened save by the presence of their specific prey. Once as he stumbled along he felt a tendril flip round his neck. He caught it and looked up. From a tangle of creeper above him waved a cluster of tentacles. He took the tendril in his hands and tore it away. Fine spines in the under side of it had left a series of bleeding punctures in his skin. In terror he gazed up, expecting the other winnowing arms to close around him. But the moment the creature, which he could now see was some kind of land octopus, felt the unfamiliar contact of the human hand tearing it away, it was seized with a contractile reaction and curled itself up into a ball.
Eventually he makes it back to his side of the Belt and encounters a team of fishers, who are leery of him since he came from the forbidden jungle. They take him on their ship back to the mainland, and from there he's taken by car to the capital, The Place.
He's brought before a court, where he's judged guilty of traveling into the Belt and sentenced to death. However, he gives a priest the pearls he gathered at the start of his adventure and is declared to be repentant.
After his body is cleansed of Belt diseases using radioactive stones (that also leave him blind), his new sentence is to become a sort of traveling object lesson, going from town to town reciting the very saga you just read (although leaving out the pleasant parts with the woman and all the pearl-bearing oysters) in order to prove the moral righteousness of scriptural law. His story, later dubbed "The Divine Tragedy", becomes widely known and the subject of high-school exams. Thus ends the saga of Ailuck.
There's also a short epilogue where Martian astronomers who have been watching the changes on the Earth through their telescopes have a heated debate over the cause -- a debate that ironically threatens to divide them into two warring camps.
I also found the titular story, "The Lost Cavern", interesting as a sort of proto-chupacabra tale:
A world-weary speleologist finds and explores a large cave system somewhere in Mexico where he's abducted by intelligent, man-sized bats, to whose society the caves are home. For weeks he is held by the "man-bats" (or Vir vespertilio as he binomializes them) and put under the tutelage of one of them, who uses drawings to explain man-bat physiology and culture. They become friends of a sort. Eventually, after being brought to the surface to witness a nocturnal, aerial dance ceremony near a volcanic vent, he is allowed to leave. On a later trip he learns that the volcano erupted and he can no longer find the Lost Cavern.
The story has elements similar to chupacabra reports (only it predates the supposed earliest chupacabra sighting by 48 years): Villagers near the cavern are fearful of the man-bats, calling them devils. Besides looking the part, a major reason for this demonizing is that the man-bats steal milk from the villagers' cattle and goats. When the speleologist asks a boy where all the village's cattle have gone, the boy replies "the devils sucked them, milk and blood". It's later revealed that they don't drink blood, instead subsisting on milk, cactus fruit, larvae, and spider eggs, and it was the villagers that let the cattle die -- these are peaceful, thoughtful "goat suckers".
Also, oddly, a "land octopus" is mentioned in this story too, but only as a red herring; when the speleologist is first grabbed from behind by the man-bats, he suspects it might be a land octopus because he was grabbed at multiple spots.
Unfortunately I didn't find much of interest in the other two stories, both purely religious in tone instead of sci-fi, so they only get one-sentence synopses:
"The Cup" -- An art thief/forger plans to steal a bejeweled cup from a vicar, only to end up helping with an exorcism and reforming his ways.
"The Chapel of Ease" -- A civil-servant discovers an ancient, seemingly abandonded chapel that becomes a refuge from his war-time office job, only to witness a recently deceased priest presiding over a ghostly congregation of dead lepers, criminals, and suicide victims on Halloween.