I can't speak in detail to the John Quincy Adams part of your question, I'm afraid, but I can sketch in some of the background to your query: the notion that we live on a hollow earth, or at the very least one that is extensively honeycombed with giant caverns and cave systems and inhabited by the dead or, perhaps, by not entirely human beings.

Walter Kafton-Minkel, one of the few reliable chroniclers of this strange belief, observes:

A world within a world is one of the most archaic concepts in world mythology, part of the archetypal image of Gaia, the Earth Mother. Many of our distant ancestors told or heard stories of their distant ancestors germinating in the dark cavern-wombs of the Earth Mother, and being born into the bright, cold world of the surface. They knew they would return again to the Earth Mother’s arms again one day in death...[and feared] being reabsorbed too soon into those dark places, where all sorts of dim ‘things’ crept.

In the last two centuries, this old idea of an underworld has been supplemented by an altogether more radical one, based on the ideas of an early nineteenth century American soldier named John Cleves Symmes who evolved a system called the ‘theory of concentric spheres’. Symmes’s central idea was that earth (and indeed every other planet in the universe) was hollow, with a crust no more than a thousand miles thick, ‘habitable within’, and that access to the interior might be gained through large holes situated at the north and south poles. Inside the earth, he predicted, smaller hollow globes would be found rotating. Symmes sought to prove this theory by leading an expedition to the northern hole (this was nearly a century before the supposed conquest of the pole by Robert Peary), with the explicit intention of claiming the interior for the United States, and this is where Adams enters the story, since he was the President who proposed backing the venture – though apparently for scientific reasons as much as geographical ones. Unfortunately for Symmes, but probably fortunately for the safety of his would-be companions, he was never able to raise the necessary funding. He died, exhausted from constant lecturing in support of his ideas, in 1829.

Though there were enormous problems with Symmes’s hypothesis – not least the remarkable implications it had for gravity, the theory of planetary formation, and geography (he claimed the northern polar opening was 4,000 miles in diameter, so big that it would swallow much of Canada, Greenland and Siberia) – the notion of a hollow earth has proved surprisingly tenacious in the years since his death. Various theorists elaborated on it (one came up with the useful notion that a small ‘sun’, some hundreds of miles across, floated at the centre of the earth, providing much-needed heat and light to the inhabitants of the interior), but it was not until the end of the nineteenth century that the next significant step was taken when the idea of a hollow earth was enthusiastically taken up by some of the occult societies that flourished at the time. Groups such as the theosophists (led by the remarkable Helena Blavatsky, a former circus bareback rider who single-handedly conceived many of the central tenets of modern occultism and the New Age movement) taught that the interior of the earth was the domain of the Secret Masters – benevolent sages possessed of enormous occult powers who guided the fortunes of mankind. From there it was only a short step to linking the hollow earth theory to the Tibetan legend of an underworld city, called Shambhala, ruled over by the King of the World, from which the destiny of mankind was closely controlled.

During the Second World War, another alternative hollow earth doctrine surfaced, in unlikely fashion, in the pages of the American pulp science fiction magazine Amazing Stories. This one was the work of a man named Richard Shaver, who promoted it by writing a series of astonishing accounts of his adventures in a vast network of underground caverns which, collectively, had a greater area than all the land masses of the surface. Shaver explained that his contact with earth’s secret inhabitants had begun when he heard strange voices calling him from out of thin air while he was working as a welder on an assembly line. Later he had been guided to one of the many entrances to the underworld by a beautiful inner earth girl who rescued him from a hospital where he had been incarcerated to recover from either injuries or a mental breakdown (Shaver gave both versions at different times). Backed by Amazing’s enterprising editor, Ray Palmer, the one-time welder insisted that his stories of the inner world were all true.

It hardly seemed likely. Shaver’s tales were amongst the wildest ever spun, even in the pages of the pulp science fiction magazines of the period. According to him, the hollow earth was populated by two races: the benevolent teros and the malevolent – and unfortunately much more numerous – deros. Both peoples were supposedly descended from an Atlantaen super-race which had abandoned the surface thousands of years earlier when a sharp increase in solar activity made it uninhabitable. Confined to underground caverns hollowed out by the Atlanteans’ advanced technology, the tero succeeded in maintaining some sense of discipline, while the dero abandoned themselves completely to vice. Some lived their lives in a perpetual debauch, luxuriating in ‘sex rays’ produced by so-called ‘stim machines’. Others indulged in torture, luring human women from the surface, penning them, raping them and then flaying or roasting and eating them when their usefulness was at an end. (Sado-masochism was one of the prominent themes of Shaver’s writings.) The Deros’ other great pleasure was causing trouble in the surface world with the help of further items of strange weaponry which caused plane crashes and other accidents, and could even be used to boil an unfortunate victim’s brain alive in its own cranial fluids. In short, Shaver charged that the evil dwarfs were responsible for almost every misfortune visited upon humankind.

Hardly surprisingly, critics of the ‘Shaver Mystery’ were quick to point out both that its author was suffering from several of the classic symptoms of paranoid schizophrenia, and that many of the letters that poured into Amazing recounting personal experiences that backed up the author’s stories patently came from the sort of people who would otherwise spend their time claiming they were being persecuted by invisible voices or their neighbours’ dogs. For example, one woman alleged she had been in a lift in the basement of an office building in Paris when she had pressed the ‘down’ button by mistake: