So why are these cities so overwhelmingly large? And where do they come from? Here's a list, by region:

North America:

The city of North Am (in Magnus Robot Fighter) does just what it sounds like — it covers almost the entirety of North America, giving you lots and lots of space in which to (what else?) fight robots.

The Maze is a huge network of underground parking garages that stretches all the way from New York to Los Angeles, in the movie Circuitry Man.

Lots and lots of SF stories predict a huge swathe of city stretching along the East Coast of the United States. One of the most famous is Judge Dredd's Mega-City One, which eventually stretches all the way down to Florida.

In Neuromancer and other books by William Gibson, a mega-city stretching from Boston to Atlanta is known as the Boston-Atlanta Metropolitan Axis (BAMA) or The Sprawl.

In He, She And It by Marge Piercy, the urban megalopolis that stretches from the former Boston to the former Atlanta is called The Glop.

And similarly, in the novel The Rise Of The Conglomerates by Thomas Nevins, a huge sprawling "Conglomerate City" occupies most of the East Coast of the United States.

There's also BosWash, the city that stretches from Manchester, NH to Virginia Beach, Virginia. It was first predicted in the 1961 book Megalopolis: The Urbanized Northeastern Seaboard of the United States by Jean Gottman.

The City in Transmetropolitan is commonly believed to be a megacity including New York and stretching as far West as the Great Lakes, which are referred to as its Western lakes.

The Greater Chicago Industrial Zone: In Halo, the former city of Chicago now covers the former states of Illinois, Wisconsin, Indiana. And Chicago is no longer really part of the United States — the people in this city-state consider thesmelves citizens of the United Nations.

In real life, some urban planners talk about an area called ChiPitts, which comprises Chicago and Pittsburgh, and everything in between.

Texarkana in A Canticle For Leibowitz, appears to cover a huge chunk of the former Texas and Arkansas, and becomes the capitol of an empire that rules the Western Hemisphere — and eventually wipes out its main rival, New Rome. (Map from Wikipedia page.)

Texas City, in the Judge Dredd comic, covers a huge area of the former Southwest — including Texas, of course.

Bay City is a massive conurbation covering San Francisco as well as its outlying areas, in Richard K. Morgan's Altered Carbon.

San Angeles appears in many different works of fiction, and it usually encompasses Los Angeles, San Diego and sometimes Santa Barbara. It's the setting for Demolition Man.

Mega-City Two also accounts for five thousand miles of California coastline — or it did, until it was nuked — in the Judge Dredd comic.

South America:

Sao Paulo/Rio: In Ben Bova's Mars, the rural poor stream into the cities of Sao Paolo and Rio De Janeiro in such huge numbers, the two cities grow into "a single urban megacity more than three hundred kilometers wide, that stretched from the beaches to the inland hills, sparkling high-rise towers for the rich, sprawling filthy slums for the poor, and smoggy lung-corroding pollution for all."

Ciudad Baranquilla, aka Banana City, is the mega city that covers most of Central America in the Judge Dredd comics.

Europe:

Greater London — in Sunstorm by Arthur C. Clarke and Stephen Baxter, London has grown outwards massively, swallowing up tons of villages and formerly independent towns. Clarke and Baxter describe London as spreading out, "kilometer upon kilometer of houses and factories... the scattered, helpless city that lay helpless below" a passing airplane.

Edinburgh/Glasgow — it's not strictly speaking science fiction, but there's a lot of talk about these two Scottish cities combining into one megalopolis in the coming century. The two cities could soon be linked by a high-speed maglev train. But it doesn't appear that any science fiction authors have written about EdinGow yet.

Metropia, in the animated film of the same name, is a massive network of subway systems and "undergrounds" linking all the cities in continental Europe. The world is running out of oil, so the leaders come up with the plan to link all of the subway systems into one huge network — which appears to be haunted.

City Europe, in the Chung Kuo series by David Wingrove, covers an enormous area of continental Europe, from Scandinavia to the Mediterranean. The city is made up of a series of "stacks" with the richest people living on the top levels and the poorest down in the wastelands below.

The south of England is occupied by Brit-Cit in Judge Dredd. Plus East Meg One is another mega-city in the Judge Dredd universe, which covers a big chunk of the former Soviet Union, including Moscow.

And of course, there's East-Meg One, the Soviet mega-city in Judge Dredd, which sprawls around the remains of Moscow — until it gets destroyed in a war with Mega-City One.

Africa:

Pan-Africa is a continent-wide quasi-state comprising several mega-cities in the Judge Dredd universe: they include Umar (the former Libya), Simba City (Cameroon), Luxor (Egypt), New Jerusalem (the northeast of Ethiopia), and Casablanca.

Gauteng is another one that doesn't appear to have popped up in science fiction very much, but it's talked about a lot in real life. In a nutshell, Johannesburg (a city already growing way past its capacity) joins up with Pretoria/Tshwane and a number of other municipalities, to form a single megacity. There are already plans to join them via a high-speed "Gautrain."

Asia:

Mega-Tokyo in Bubblegum Crisis. An earthquake splits Tokyo in two, and as the city rebuilds, it gets even larger and much more sprawling, coming to be known as Mega Tokyo. Here's a map of Mega Tokyo, from B-Club Special (via Igarashi) Likewise, Akira takes place in Neo Tokyo, a sprawling metropolis of steel and neon. And the anime Cyber-City Oedo 808 takes place in a fictional future "Edo," or Tokyo, which is apparently much larger than the existing city.

And real-life urban planners talk about the Taiheiyo Belt, which will cover the Pacific coast of Japan including Tokyo, Osaka and Nagoya.

A single continuous robotic structure known as The Host covers almost all the islands of Japan, and 50 million people live inside it, in Magnus Robot Fighter and Rai.

And of course, Judge Dredd does not leave Asia untouched — Hondo City covers most of Japan, from Hokkaido all the way down to Wakayama.

Australia:

Greater Sydney is predicted to encompass a region spanning from Melbourne, all the way up to Queensland along the coast. But as with Edinburgh/Glasgow and Gauteng, it doesn't appear that anybody's written science fiction about this megalopolis yet.

The South Pole:

A continent-wide city called Antarcto covers the whole of the Antarctic, in Magnus, Robot Fighter. Because robot-fighting is best served... cold.

And of course, the city of Holy Terra, or just Terra, occupies almost the entire planet's surface in Warhammer 40,000.

Additional reporting by Alexis Brown. Map layout by Stephanie Fox.

Said Piercy, the impulse that drives us to imagine a utopian future is the fact that "You have to have a way of imagining a society you want you or your children to be able to live in." On the other hand, dystopian fiction asks the question: "If this goes on, what is going to happen?"

In other words, one is more based on our hopes for our children's future. The other is based on our actual observations about the way the world is, and how it appears to be developing. [UNM Today]

Of course, plenty of original movies do get turned into books - but they're usually rushed novelizations, written in a month by someone who's juggling ten other deadlines and adding speech tags to the movie script. If you're lucky, you get a few extra insights into the characters and one or two scenes that the adapter added, or which were cut from the movie before or after filming. Plus, of course, the movies that get their own book adaptations aren't usually ones which could benefit from a really smart dose of storytelling. Movie adaptations of books, meanwhile, are usually disappointing for a whole different set of reasons.

But every now and then, a movie comes along down the pike that actually cries out for a smart, interesting book that brings out the ideas simmering below the surface. Here are ten movies that I'd love to see a really smart book version of, and the authors who would write them in my fondest dreams.

Twelve Monkeys. Cole (Bruce Willis) travels back in time from a plague-ravaged future to try and discover the source of the virus, but he ends up tangling with his own past in unpredictable ways. I was torn between listing this one and director Terry Gilliam's other dystopian epic, Brazil. But of the two movies, I think I'm more desperate to read a really thoughtful novel of Monkeys, preferably written by someone who watched the film with Gilliam a few times. There's so much confusing stuff in this movie, especially Cole's causal loop - is he creating his own dystopian future, or is he simply trapped in the logic of already-existing events? Did the scientists send Cole back on purpose to make sure their plague-ridden timeline "happens," as some have suggested? (In which case, why would they be worried about that, given that it's already happened?)
Who should write it: Marge Piercy, author of Woman On The Edge Of Time. She knows all about time travel, madness and the long reach of dystopia.

The Fountain. Meredith suggested this one - there's already a graphic novel adaptation of Darren Aronofsky's original screenplay, the one he never got to film. But there's no prose novelization of the actual movie, which I found to be a huge let-down despite its sprawling, ambitious plot. Judging from the results of our recent poll, many of you consider The Fountain an underrated masterpiece. Maybe a book could flesh out some of the confusing stuff about the present-day cancer cure and just what's going on with that weird tree-in-space sequence.
Who should write it: I'm going to go with Jonathan Lethem, author of Motherless Brooklyn. He might be able to ground the present day stuff and add some life to those lifeless characters, and when he's channeling Philip K. Dick, he does weird-and-fantastical quite well. Maybe it would all feel epic and personal, the way I think the film was supposed to.

The Brother From Another Planet. John Sayles' story of an escaped slave with weird feet who lands up in present-day New York is one of my favorite films, although I haven't seen it all the way through in a decade. Joe Morton is fantastic as the mute escapee, who has a strangely close relationship with technology.
Who should write it: Tobias Buckell, author of Sly Mongoose, has dealt with themes of slavery and alien cultures in a lot of his writing.

Sleeper. Wikipedia claims this film is loosely based on the H.G. Wells novel The Sleeper Awakes, but I would say "loosely" is the operative word. And this is such a crazy slapsticky subversive novel, complete with humans impersonating robots, Orgasmotrons, a fake utopia and nose-cloning. And so much more.
Who should write it: Douglas Adams, if he was still alive? Actually, I'm going to go with io9 contributor Austin Grossman (Soon I Will Be Invincible), just because I think he could nail the neurotic Woody Allen tone, while doing a lot to flesh out the absurdity of this freaky dystopia.

Possible Worlds This little-known film stars Tom McCamus as a man who keeps journeying through different alternate universes and having a relationship with the same woman (Tilda Swinton), which always seems to end badly. And then there's a twist, which I won't reveal here but which we gave away in a found footage.
Who should write it: Audrey Niffenegger, author of The Time Traveler's Wife.

S1m0ne. Andrew Niccol's most disappointing film totally deserves a novel told from the point of view of Al Pacino's character, a third-rate movie director who creates a virtual actress to save his troubled movie - and then has to deal with her becoming a superstar. A novel might be able to make the movie's premise more believable and dispense with some of the VFX problems that dog the movie, and a tight focus on Pacino's POV would allow us to probe the psychology of a man who brings to life an irresistible virtual avatar, in a cross between Pygmalion and Cage Aux Folles.
Who should write it: Amy Thompson, author of Virtual Girl, who manages to make that novel's skeezy programmer who creates a gynoid and then tries to enslave her actually sympathetic.

The Matrix Trilogy. No, not just the first movie. I want to see the whole trilogy as one sprawling, insane novel about cyber-avatars. I want all of the lame discussions about free will in the second movie and all of the lame everything in the third movie to be beaten into submission, and the whole disappointing mess transformed into a seamless whole, the story of humans trapped in a virtual world rising up against their machine overlords, while a virtual man-in-black becomes a megalomaniac.
Who should write it: That's the hard part. There are so many cyberpunk authors I'd like to see try their hand at it. But in the end, I'm thinking Charles Stross.
He does sprawling post-human stories really amazingly well, and might add a whole extra conceptual layer to the Wachowskis' somewhat facile world-building.

Primer. This knotty time-travel movie actually stands on its own remarkably well, but I'd still like to see a smart, thoughtful novel that deals with all the of the intersecting timelines and unraveling protagonists.
Who should write it: David Gerrold, author of The Man Who Folded Himself, still possibly the weirdest time-travel novel of all time.

Slither. You might think this is just another over-the-top body horror movie, about alien parasites who infect a town's residents. But this movie goes so much further, showing how a woman can't escape her abusive husband. The parasite infects her husband first, and then all of the people whom it infects afterwards speak with the husband's voice, so she's constantly trapped. It's up there with Society and Dead/Alive in the disturbing horrific social commentary sweepstakes.
Who should write it: The great d.g.k. goldberg, if she was still alive. Otherwise, I would say Nalo Hopkinson, author of Brown Girl In The Ring.

Sunshine. The screenplay is available in book form, but there's no novelization. I loved this film, but many people don't seem to agree, and maybe a really strong novelization could help win over the doubters, especially if it made the slasher-movie third act feel like it grew naturally out of the rest of the story.
Who should write it: I'm thinking maybe Stephen Baxter, who's shown a talent for writing madness as well as planetary disasters and space exploits.

Note: I was going to include Galaxy Quest on this list - but realized it already has a novelization, by Terry Bisson. Who, by amazing coincidence, is probably exactly who I would have chosen to novelize that movie. Has anyone read Bisson's Galaxy Quest novel, and is it as good as it ought to be? It's only one cent on Amazon (plus a few bucks' shipping, of course.) Also, did you know that Christopher "The Prestige" Priest has novelized David Cronenberg's Existenz?