What accounts for the Sphinx's inscrutable smile? Why is this colossal monument riven with deep vertical fissures that could only have been caused by thousands of years of heavy rainfall not a notable feature of Egypt's climate since long before the pharaohs? Does the secret lie in the ruins of a lost civilisation, two miles beneath the ice of the South Pole?
This possibility is being entertained by readers who have pushed Fingerprints Of The Gods to the top of the non-fiction bestseller lists. Its author, Graham Hancock, has concluded after a worldwide quest that the end is nigh. In the year 2012, he predicts, the earth's crust will shift dramatically around its core, like the loose skin of an orange, bringing widespread death and destruction.
Does your taste run to global conspiracy? Then perhaps The Robots' Rebellion is the book for you. David Icke, a former spokesman for the Green Party, Coventry City goalkeeper and BBC sports commentator, writes that the world is being manipulated by a secret government that works above elected governments, engineering events to its own sinister ends. He calls for a Gandhi-type campaign of non-cooperation.
Or do you like alien sex? Stand by for Abduction: Human Encounters With Aliens, an American bestseller by John Mack, who has recounted the experiences of alien abduction victims. Women had their eggs removed and men had a long probe inserted in the anus before being forced to donate sperm.
The earnest conviction of these authors is beyond doubt. Hancock is a former East African correspondent of The Economist, who previously spent five years tracking down the Ark of the Covenant to a church in Ethiopia, where it apparently still resides. Icke took the bold step of proclaiming himself the Son of God and sacrificed his political career to pursue a higher purpose. Mack is a Pulitzer-winning Harvard psychiatrist, who has risked academic excommunication by lending credibility to unorthodox views.
The question is not why such books are written, but why they are read. Walk into any large bookshop and you will find shelves groaning with works on Atlantis, UFOs, the occult, ancient religions and the transmitted thoughts of dead Native Americans. The latest sub-culture concerns the "ruins" on Mars.
"The output has grown steadily," reports Peter Donaldson, who runs Red Lion Books, a specialist bookshop in Colchester. "It used to be the smaller publishers, but now the mainstream publishing houses are increasingly involved."
What instinct impels regiments of ordinary people to seek such of alternative explanations? "There is always going to be a number of people interested in anything that's a bit different from their run-of-the-mill life, " Donaldson says. "Some read it with a pinch of salt; others are really into it. They join the local UFO society and go to meetings."
Psychologists agree that such people are searching for meaning in life. "The more bizarre the explanation, the more intriguing it is," says Geoffrey Scobie, a social psychologist at Glasgow University. "There is a need to give cosmic meaning to our existence. People are asking questions about our ultimate purpose, which we can't determine within the system."
Dr Sue Blackmore, a parapsychologist at the University of the West of England in Bristol, believes that evolution's apparent lack of purpose can prove distressing to some people. "They want there to be someone out there who has created a reason for suffering and doing good rather than evil." By pursuing the mystery of such phenomena as crop circles through pseudo-science, they are emulating scientists without the travail of study, she believes.
The trend also points to a fundamental dissatisfaction with the explanations offered by historians and scientists. There is still no tangible proof of Darwin's theory of evolution by natural selection. With each archeological find, the mystery of man's origins deepens. People suspect the human body is more than a machine that can be controlled by drugs. Why do we possess such enormous surplus brain capacity?
The answers cannot be found in our own indigenous myths and folklore, which have been largely eradicated by our fractured development. We look to authors who can entertainingly extrapolate coherent theories from the surviving records and oral histories of ancient societies, scrambling them with modern knowledge.
This rise of "irrationality" infuriates Michael Allaby, who issues a broadside on behalf of orthodox scientists in his forthcoming book, Facing The Future. Appalled that half the people participating in a British survey said they believed in faith healing and one third consulted astrology charts, he complains that our culture is dominated by unscientific ideas, many of which have a dubious claim to antiquity.
Yet the publishing sensation of 1988, suggesting a divine hand in the creation of the universe, was written by an eminent scientist. A Brief History Of Time, by Professor Stephen Hawking, Lucasian professor of mathematics at Cambridge, posited an infinite and eternal universe, in which the numbers governing the laws of science seemed to have been "very fine ly adjusted" to make life possible.
A dramatically different interpretation has been made by a no less distinguished scientist. In his 1983 book, The Intelligent Universe, the physicist Fred Hoyle argued that a dying alien species had seeded the universe with interstellar bacteria, which became the building blocks of life when they reached the assembly station of earth.
In 1979, James Lovelock, a Nasa scientist preparing for unmanned soft landings on Mars, inadvertently spawned an earth goddess cult. In Gaia: A New Look At Life On Earth, he suggested that our planet was a self-regulating mechanism in which living organisms stabilised global climates. Despite his protestations, Gaia came to be embraced as a guiding intelligence.
If Hoyle and Lovelock could arrive at such hypotheses, what was stopping anybody from giving full rein to speculation?
In his 1968 blockbuster, Chariots Of The Gods, Erich Von Daniken paved the way by fusing together the elements that continue to excite most interest: manipulation of man's evolution, alien abductions in a breeding programme, a reinterpretation of biblical history, and the erection of giant monuments by advanced technology. Von Daniken mustered impressive evidence of flying machines from pre-Incan relics, the Mahabharata and Tibetan books. One Mayan carving showed an astronaut in the cabin of his spacecraft, he claimed.
Fingerprints Of The Gods, a title curiously similar to Von Daniken's considering that Hancock wishes to be taken seriously, contends that the monuments of the ancient world the Sphinx, the pyramids and the edifices of Incas and Mayans were built by a race of super-beings who lived 15,000 years ago, before the last Ice Age. Their tall structures were intended as a sign to future generations before disaster engulfed them.
And what an apocalypse. According to Hancock, most of the super-beings perished on the enormous landmass of Antarctica when a shift in the earth's crust dragged them from temperate latitudes into the polar regions. He claims their legacy is contained in the dimensions of Great Pyramid, which give precise read-outs of the equatorial circumference of the earth and its polar radius.
He has built on the work of the construction engineer Robert Bauval, whose 1994 bestseller, The Orion Mystery, claimed that the layout of the three pyramids of Giza was designed to mimic the pattern of Orion's Belt exactly as it looked in 10,500BC. Using an astronomical computer to show the positions of the stars in that age, he discovered that one of the shafts leading upwards from the Great Pyramid would have pointed straight at Orion, identified with the Egyptian divinity Osiris.
All eyes swivelled back to these shafts late last year when a German engineer, Rudolf Ganterbrink, was banned by the Egyptian authorities from continuing his attempts to send a small robot scurrying down one with a video camera. There was fevered speculation that behind a huge slab blocking the tunnel was a concealed chamber. This mystery alone is good for half a dozen new books.
But it is intrusion of another kind which now holds many in thrall. Foreshadowed by HG Wells and Jules Verne in the last century, imprinted on our minds by Orson Welles's shock radio broadcast in 1938, the aliens invaded in force during the 1980s, intent on close encounters with humans. Thousands of people reported alien abductions, 90% of them in America. One self- proclaimed victim, Whitley Streiber, made a killing with his controversial book Communion.
The cover-up theory took hold with Timothy Good's book, Above Top Secret, in 1987. Good, dubbed "the little green Menuhin" when he was a violinist with the London Symphony Orchestra, reproduced a document purporting to be an "eyes-only" briefing for president-elect Dwight Eisenhower on the recovery of four aliens and the wreckage of their crashed spaceship from New Mexico in 1947. He has extended the theory with Alien Liaison and Alien Update.
The puzzling thing about aliens is that each nation has its own type. The Americans are generally abducted by short, grey-skinned beings with domed heads. The French encounter mythological, gnome-like entities. The British are interfered with by tall, blonde, Nordic types like Joanna Lumley.
Jenny Randles, co-author of Science And The UFOs (1985), says that under hypnosis witnesses relive a genuine trauma, which is made to fit a visual preconception. Randles, a sceptical investigator who formerly worked with the British UFO Research Association, believes that many trauma cases are activated by unidentified atmospheric phenomena (UAP) which exert a "mind- scrambling" effect on the brain, releasing hallucinations from deep in the human pysche.
Such rational theories are not popular. "The books that sell are those that seem to suggest we are not the only form of life and that our achievements are manipulated by others," she says. "Conspiracies attract many more readers."
She is worried by the trend. "In the last five years there have been half a dozen books about UFOs which have made in excess of Pounds 250,000. Veracity is no longer a measure of whether a book should be published. Some have been good, but others are being engineered and they don't deserve to be published."
Perhaps it is time to return to first principles. The world's oldest legend probably belongs to the Efe Pygmies, the most ancient surviving stock of Homo sapiens, who live in the equatorial forest of Africa. They tell of a cataclysm that annihilated the world's great cities and teeming populations. Associating this disaster with an ecological sin, symbolised by picking the forbidden fruit of the tahu tree, they renounced civilisation, and opted for the most simple lifestyle, sustained by a lofty monotheistic religion which tells of another cataclysm and the second coming of a saviour. Long before the missionaries arrived, they prayed to "Our Father" and said grace before meals. At least, so claimed Jean-Pierre Hallet, a bush sociologist, in his 1973 book, Pygmy Kitabu.
Hallet argued that the Ituri Forest in Zaire was widely identified in the Old World as the Garden of Eden. In his view, the fall of man was a true story, which degenerated into the superstitions and dogmas that mankind dignified with the name religion. Deified by the Egyptians, the pygmies were the source of the world's most abiding legends, and their tongue was the linguistic root of languages.
What does our reading matter say about our intelligence? And what do aliens read? A science fiction story recounts how the human race struggled to establish the intentions of some occupying aliens. Finally, they deciphered the title of an alien book: "How to Serve Man". Their relief was tempered by the text. It was a cookery book.