Menacing shadows stalk Gormenghast; wizards and ghouls duel in Middle Earth; and deep in the sea Beowulf and Grendel's mother tussle for supremacy. These other worlds are classics, literary scenes we are used to, places which are solid with the weight of detail. They are inhabited by beings such as ourselves, but yet where the structure of life, for all its twists and turns, is predictable. Evil, on the whole, does not triumph there. And if it does, it won't make the evil-doer happy.
A crop of recent publications mine this fertile field anew, most notably Philip Pullman's Northern Lights, which recently won the Carnegie Medal. It is a field which, as Mr Pullman says, allows enormous freedom to the writer who can, like John Milton in Paradise Lost, embody psychological states and moral problems in the flora, fauna and landscape of an imagined world.
It is surely significant that adolescence is the time when the alternative universe has greatest appeal. Books like Elidor by Alan Garner or the Dragon Fire trilogy by Charles Ashton (recently republished by Walker) with their stress on quests, tasks and ordeals, with the sense of a cosmos governed by fixed yet moral rules whose workings must be understood very quickly in order for protagonists to survive, are the direct descendants of fairy tales, with their arbitrary moral consequences.
According to Bruno Bettelheim in The Uses of Enchantment, one of the main themes in fairy tale - the journey of the young person through the dangerous forest to solve tasks and gain rewards - is the story of adolescence itself. Add a lot of techno-fun, a playful rather than a formulaic use of language and you have A Wizard of Earthsea by Ursula le Guin, The Owl Service by Alan Garner, the Discworld series of Terry Pratchett or The Talking Head (new from Orion, Pounds 3.99) by Elizabeth Pewsey.
There are also Pratchett's tender, moving Johnny Maxwell books. Johnny and the Dead, Only You Can Save Mankind and Johnny and the Bomb do not deal so much with alternative worlds as with parallel ones. In Johnny and the Bomb, young Johnny finds himself at a crossroads of possibility where picking the right path will undo the deaths of a street full of people in the Second World War. Here the story exploits a structure which, says the author, allows the reader to think that "in this world it's just possible to get it right".
As more of the essentially random, amoral character of the "real" world is revealed to young minds, more intensely springs the desire for a forum in which the odds are not weighted against the gladiator with the brave sword. In the parallel universe there is the chance - as in Robert Harris's Fatherland - to rewrite history, albeit as a dystopia.
Parallel and alternative worlds can overlap, as they do in Northern Lights. On the one hand, there are the bears with armour and the magical instrument which finds the truth (now that would really be useful). On the other, there are recognisable versions of the Oxford University and Catholic Church of our world, given a satirical spin.
Perhaps unsurprisingly, Mr Pullman's favourite parallel universes are Herge's Tintin country, "where the surface looks the same as in our world but the action is endlessly, inventively comic", and Tove Jansson's Moominland, where the "people are the same inside, but their outsides are comic".
In more wholly alternative universes, though there is rarely any explicit religion, morality permeates existence. Take C S Lewis: while his adult fantasies, beginning with Out of the Silent Planet, enabled him to work out other versions of the modern human dilemma, his children's novels, beginning with The Lion, the Witch and the Wardrobe, allowed him to create a world where an eternal moral struggle is physically enacted.
But alternative universes are safer. Terry Pratchett believes that The Lord of the Rings, for example, is "a lot safer, despite all the dangers, than this world because it was created by a scholarly don in a study who'd read all the books and was a bit more reliable than God".
No child inveigled home by Sauron, Dark Lord of Tolkien's Middle Earth would ever mistake him for someone sent by their mother; the Morrigan, in Alan Garner's The Moon of Gomrath, signally fails to entice children into her car; wicked Mrs Coulter in Northern Lights cannot deceive her canny young daughter. Whereas, as Terry Pratchett says, "in our world the underlying structure is either incredibly easy to find or incredibly difficult", in the alternative universes you know where you are: generally in for a rough ride but a happy ending.
Look at Star Wars, described by Mr Pratchett as "a classic fantasy with space stuck on the outside". Here is a young farm lad, low on the totem pole. Aided by a fair bit of luck - or is it more than just coincidence? - he finds his way to the top and gets his just reward. As young people say: in your dreams.
Look at Terry Pratchett's invigoratingly comic Discworld series in which all kinds of earthbound necessities get stood on their heads. Look at Watership Down by Richard Adams, in which cuddly animals survive - or some of them, at least.
We all need dreams. Some of our dreams may be a bit tacky, several descendants down from the shining originals like The Lord of the Rings. Some may be nightmares, impenetrably Gormenghast-like. But even if they do come off the sword-and-sorcery shelf or the animal-odyssey shelf (sons of Watership Down) or the small-creatures-against-a-hostile-world shelf (daughters of the Little Grey Men and the Borrowers) or the secret-life-of-animals shelf (descendants of Ratty, Mole and Toad) or even the old-fashioned but still active life-beyond-death shelf (ancestor: The Water Babies by Charles Kingsley) they continue to tap in to that primal world of the imagination which gathered our ancestors around campfires and still glues us to the telly.
Thus recently we have Foiling the Dragon by Susan Price (Point Fantastic Scholastic Pounds 3.50), a sub-Pratchett fantasy in which, with a degree of humour, a pub-poet has to save his life by rhyming his socks off against a versifying worm. Louise Lawrence's Llandor trilogy (Collins Pounds 8.99 each) has strong echoes of Le Guin's Earthsea, with initiates Carrie, Roderick and Craig, albeit with cockney accents, doing whatever it takes to save Llandor from black magic.
In her continuing Snow-walker's Son trilogy, Catherine Fisher's The Soul Thieves (Bodley Head Pounds 9.99) and The Empty Hand (Red Fox Pounds 3. 50) use Nordic imagery to freshen up her icy tale: shades of Hans Christian Andersen's Snow Queen mixed with Rosemary Sutcliff's Vikings. In Firefly Dreams by Jenny Jones (Point Fantasy Pounds 3.99), two children have to find their missing cousin in a dangerous, alien city, similar to the tasks faced by Alan Garner's children in Elidor and The Moon of Gomrath.
And in Wyvern's Fall by Toby Forward (Penguin Pounds 4.50) "real people" bicker and fall out in a satisfactorily realistic way while at the same time strange and whirring universes swoop around them - Garneresque once more. Bickering, which gives Elizabeth Pewsey's Talking Head most of its vitality, is an essential part of a good alternative world read. It gives most of its vitality. Children, after all, do quarrel a lot.
The recent survey of children's reading habits by the Roehampton Institute Children's Literature Research Centre (TES, September 6) showed that the single most important factor influencing children's choice of book was identification with the behaviour and predicaments of the characters. This far outweighed any other factors such as identification by race and gender.
No one understands this crucial factor so well as Terry Pratchett, whose Discworld series, soon to add Hogfather to its long list of titles, breaks up its diet of theological speculation (in Small Gods), political satire (Interesting Times) and speculation about fairy tale and fantasy itself (Witches Abroad and Lords and Ladies) with splendid spats, quarrels and insults from assorted witches, dwarves and warlocks. Even Death, speaking in capital letters, is not above a line of dry humour.
And therein lies perhaps the secret of the alternative world. It is one in which death has partly lost its sting and the grave has only a qualified victory. To use another phrase beloved by young people today: dream on.