In examining the wreckage and the dead, Dyer's party find that some of the dead bodies have not just been injured by flying wreckage or frozen by exposure; they've been sliced open. Lovecraft does not use the word dissected, but that is evidently what has happened. They also find that the six damaged specimens have been buried in the snow underneath star-shaped mounds, like the soapstone pieces, and a lot of strange triangular footprints in the snow. The eight intact specimens of the star-headed creatures, are missing; so are three of the sleds, one of the sled dogs, and an odd assortment of supplies. Gedney, one of Lake's party, is also missing. The inference is inescapable. Lake's party must have gone mad.
Right? Isn't that what you were thinking? No? Maybe you've watched too many monster movies.
Dyer decides to attempt a flight across the mountain range. Because the range is so high, he will be unable to take the plane fully-loaded, so he brings only Danforth, one of the expedition's unpaid interns, with him to pilot the plane and what equipment they think absolutely necessary. As they approach the mountains, like Lake's survey before them, Dyer notes the peculiar outcroppings of rock on some of the mountain slopes. Dyer, the geologist, is reminded of the "Giant's Causeway", a natural rock formation in Ireland, but also of the ruins of Machu Picchu in Peru. Higher up, they encounter a misty haze, which Lake had earlier mistakenly guessed was smoke from volcanic activity.
Threading their way through the impossibly high mountains, they navigate the lowest pass they can find, and on the other side of the mountains they find something truly startling: A vast ruined city, stretching across a wide plateau. Dyer realizes that this is the same city they saw in the mirage which appeared when they first approached the mountains. The image of the city had been reflected off a layer of ice particles in the upper atmosphere; a common enough phenomenon in the polar regions. Flying over the city, Dyer is struck by the recurring star-shaped motif in the buildings and plazas of the labyrinthine city.
They land their plane and begin exploring some of the buildings. They find several chambers with elaborate carvings and from these carvings they are able to piece together a history of the city's inhabitants. The carvings confirm what the reader has already guessed; that the city was built by the strange star-headed creatures whom Lake had dubbed the "Elder Ones" after creatures mentioned in the Necronomicon.
This part I find to stretch my willing suspension of disbelief a bit, that they could reconstruct a detailed history of the Old Ones after studying some decorative art for a few hours; but Danforth has read the Necronomicon from cover to cover and the images they find confirm the stories in that tome. Everyone who goes to Miskatonic University seems to have some sort of familiarity with the Necronomicon; I suspect that they make fraternity pledges read a page as part of their initiation hazing.
The Old Ones began building this city eons ago, when life as we know it was just beginning to crawl out of the slime. In fact, earlier Lake had suggested the possibility that the Old Ones had actually created life on Earth as "a jest or a mistake." For hundreds of millions of years, they ruled the planet as geological ages passed and the continents shifted. Lovecraft cites Wegner's Continental Drift Hypothesis, which at the time was regarded as highly speculative. The Old Ones built most of their cities underwater, with the aid of artificial creations called Shuggoths: amorphous blobs capable of changing form to whatever shape was necessary and of obeying the Old Ones' hypnotic commands; but built other colonies on land, such as the ancient city in the Antarctic mountains.
Over the Strange Eons, the Old Ones found their dominion of the earth challenged by other beings from Other Worlds; the Mi-Go, the Spawn of Cthulhu and the Fungi from Yuggoth. Here Lovecraft is tying the star-headed Old Ones to other Cosmic Horrors from previous tales, as August Derleth later did with his "Cthulhu Mythos." But as S.T. Joshi points out, he is also de-mythologizing the Mythos. Where previous stories called these Cosmic Entities "Elder Gods", this one makes clear that these are aliens mistakenly worshiped as gods by our primitive ancestors. One gets the impression that Lovecraft was a little embarrassed by the 'men of science' chanting spells of exorcism in "The Dunwich Horror". There is no magic in "At the Mountains of Madness".
The Mi-Go retreated to the peaks of the Himalayas; the Fungi from Yuggoth flew back to Pluto and Great Cthulhu's domains sank beneath the waves. The Old Ones remained, although diminished in power. They had lost their ability to fly through space on their membranous wings; their art showed a decline in sophistication. They had to put down an uprising by their Shuggoth slaves, who had evolved enough intelligence to revolt. As the continents shifted and the climate began to change, the now-decadent Old Ones built a new city near the old near the great abyss where they had first colonized the Earth. Eventually, the land became so cold, they had to abandon the old city completely. Something else also happened; something so terrible, that the carvings only hinted at it; someting terrible and deadly which the artists of that long-dead city refused to portray.
In this history of the Decline and Fall of the Star-Head Empire, I suspect Lovecraft is writing a parable about the America he knew. Certainly in his view American culture and the arts had degenerated since the 19th Century. More suggestive still are the Suggoths, an underclass which performed all the heavy labor for the Old Ones and who eventually rebelled. The theme of Working Class rising up to bring down Society was a common one in the early 20th Century. Lovecraft had a strong streak of xenophobia in him. Although the influence of his circle of friends in the Wide World outside of Providence helped soften his views, his sojourn in New York as a penniless writer competing for jobs with a flood of immigrant workers to a certain extent reinforced his prejudices.
Dyer and Danforth decide to continue their exploration of the City to the Abyss, shown in the carvings, where the later City of the Old Ones was built. The idea of a kind of Abyss near the Pole is another one that goes back to the 19th Century. In the early 1800s, a US Army captain named John Cleves Symmes, Jr. proposed a theory that the Earth was hollow, and contained several concentric shells which could be reached by large holes at the North and South Poles. He lectured widely on his theory and unsuccesfully lobbied President Andrew Jackson to send an expedition to the polar regions to find it. Edgar Allan Poe's story "MS. Found in a Bottle" ends with its protagonist sucked down into such a polar abyss, and Poe used some of Symmes' theories in his novel The Narrative of Arthur Gordon Pym, which Lovecraft alludes to in "Mountains of Madness".
Making their way through the labyrinthine passageways under the City, Dyer and Danforth encounter some of the weirder creatures to ever appear in a Lovecraft story, and these are completely mundane: flocks of six-foot tall albino penguins. Living as they do underground, they have evolved to be sightless as well as flightless and have no eyes. Although he does not explicitly say so, I suspect these are meant to evoke the mysterious white shrouded figure which appears at the end of Arthur Gordon Pym. The penguins are harmless, but majorly creepy.
Creepier still is what the two find next: the tracks of the sleds missing from the Parks camp, and the lingering odor that the specimens had. Following the tracks they find the sleds themselves and a kind of camp. And in the sleds they find Gedney and the missing sled dog; both dead, but both carefully wrapped up, just like specimens.
Lovecraft has been coy about directly stating what really happened at Lake's camp, but by now it's pretty clear. The star-headed specimens Lake found were not really dead; or at least not all of them were. Under the warmth of the Antarctic sun, they revived. The dogs, maddened by their unnatural scent, attacked the creatures and lead to the horrific slaughter of the camp. After which, the creatures collected specimens of their own and gear from the camp and proceeded to the City themselves. Dyer guesses that these specimens came from an earlier era, and so the City as it is now will be almost as strange to them as it is to its human explorers. He and Danforth find, in this rude camp, sketchbooks taken from Lake's camp which the Old Ones have added drawings of their own in a peculiar style reminiscent of the carvings Dyer has seen.
Following the trail of the Old Ones through the darkness, the unpleasant smell becomes mingled with another odor, even more vile. Then they come across the bodies of four Old Ones, their starfish heads brutally decapitated and covered with slime.
They had not been even savages -- for what indeed had they done? That awful awakening in the cold of an unknown epoch -- perhaps an attack by the furry, frantically barking quadrupeds, and a dazed defense against them and the equally frantic white simians with the queer wrappings and paraphernalia ... poor Lake, poor Gedney ... and poor Old Ones! Scientists to the last -- what had they done that we would not have done in their place? ... Radiates, vegetables, monstrosities, star-spawn -- whatever they had been, they were men!Lovecraft has written disparagingly in letters about science fiction writers who give their aliens human attitudes and motivations; but here he succeeds in making grotesques abominations sympathetic as his narrator slowly comes to recognize the same attitudes and motivations he has.When they first find the bodies, Danforth gives an involuntary scream. Now, as they examine them, Dyer and Danforth hear an answering noise in the blackness beyond, a piping noise, sounding something like "Tekeli-li!" the cry of the gigantic bird-creatures from Poe's Arthur Gordon Pym. Dyer guesses that it's one of the group of Old Ones who has murdered its companions. He and Danforth begin to run.
They dash headlong in a blind panic, retracing their path. Coming to the large chamber where they encountered the penguins, they are fortunate in that the mists of the cavern and the large flock confuse their pursuer and it continues down the wrong corridor. But the two men look back and see that Dyer had been wrong. Their pursuer had not been a starfish-headed Old One, but a huge amoeba-like creature, with a multitude of eyes floating on it's formless body. It was a Shuggoth, one of the former slaves of the Old Ones, which had once again revolted against its masters -- this time successfully. The Old Ones were now all truly dead, and only their mindless slaves dwelt in their haunted city.
The two men proceed to the surface. Danforth is near hysterical, babbling the names of the Boston subway train stations as they dash through the tunnels. When they get back to their plane, Dyer takes the controls. Danforth is the more skillful pilot, but his nerves are too shot to fly.
Guiding their way back through the high mountain pass, Dyer is too busy concentrating to look back behind them as Danforth does, and so he does not see what Danforth sees.
What does Danforth see? A bigger, even more terrifying Shuggoth? The mysterious black sphinx Poe wrote of in Arthur Gordon Pym? A mirage like the one they saw before, only this time showing what lies beyond the City? The Abyss of the Old Ones itself?
Those are some of my guesses. Dyer has his own, but will not speak of them. Danforth will not speak of it at all, except in delirium.
He has on rare occasions whispered disjointed and irresponsible things about "the black pit", "the cavern rim", "the proto-shoggoths:, "the windowless solids with five dimensions", "the nameless cylinders" "the elder pharos", "Yog-Sothogh", "the primal white jelly", "the colour our of space", "the wings"...But when he actually saw it... whatever it was he saw... he only shrieked one word, over and over:"Tekeli-li! Tekeli-li!"