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.

Criminal Activity

The Time Traveler's Wife by Audrey Niffenegger: Involuntary time travel comes with plenty of disadvantages, not the least of which is finding yourself suddenly and unexpectedly naked without any money. Fortunately, the predestination paradox can be a handy survival tool. Time traveler Henry often finds himself sent to the same points in time and space as his younger self, and teaches him how to find clothing, pick locks, and steal wallets. It's sort of like illicit father-son bonding, just with himself.

The Joy of Sex

The Time Traveler's Wife: Another unexpected side effect of time travel is that a horny, adolescent Henry is every now and then confronted with a nearly equally young, equally horny duplicate of himself. This makes for some rather spectacular instances of masturbation, but it's really awkward when his father walks in on him.

—All You Zombies— by Robert Heinlein: The Unmarried Mother was an intersex, though apparently female, teenager who was seduced by a mysterious older man. Many years and a sex change later, she, now he, is sent back in time, where he meets and makes love to a very familiar girl.

The Man Who Folded Himself by David Gerrold: Daniel Eakins is the sort of time traveler who throws caution to the wind, sampling all that time travel has to offer: foiling assassinations, visiting great moments in history, and using his knowledge of the future to bet on the ponies. So it's no wonder that when he meets up with the same- and opposite-sex versions of himself, he tends to get it on with them.

Futurama: Bender's Big Score: When the alien nudists get a hold of the time travel code tattooed on Fry's rear end, they're mostly interested in stealing artifacts from 20th Century Earth, although they do at one point take a time out for Nudar-on-Nudar nookie.

How to Win a Fight

The Kid: Russel Dritz's dirtbag ways may go back to his childhood, when he was picked on by bullies and lost his mother to illness. When Rusty, his younger self, ambles into Russel's life, he finds there are some subtle ways that he can change the past. First on the agenda: Getting the kid into a boxing ring so he can learn how to throw a punch.

How to Become Rich and Powerful

Back to the Future, Part II: The 2015 version of Biff decides that all of his troubles would be solved his he had been extremely wealthy in the past. So he steals Doc Brown's time-traveling DeLorean and, with a 2015 sports almanac in hand, travels to 1955, when he gives the almanac to his younger self. And it seems to work: Biff is rich beyond his wildest dreams, he's quietly had his rival George McFly murdered, and he's married to George's now artificially-endowed widow Lorraine. Of course, it all goes to hell when that pesky Marty McFly appears on the scene.
Gargoyles "Vows:" In move that revealed the entire series as one big predestination paradox, David Xanatos travels back in time on his wedding day to give his younger self a collection of priceless gold coins, along with instructions on how to invest the proceeds from their sale. Is it cheating? Probably, but in Xanatos's mind, it makes him the very definition of a self-made man.

By His Bootstraps by Robert Heinlein: When Bob is pulled thirty thousand years into the future by a slightly older, though no wiser version of himself, he discovers that humans have become a primitive, compliant people. Diktor, a fellow native of the 20th Century, explains that a technologically advanced person could easily become king of these sheep-like folks, and gives Bob a list of 20th Century items to bring to the future. Bob complies, but travels to a point ten years before he meets Diktor. It takes Bob a shockingly long time to realize that he's in a Heinlein story and that he is himself Diktor.

How to Win the Girl of Your Dreams

Futurama: Bender's Big Score: Fry is distraught when Leela, the love of his life, is won over by an older and more mature stranger named Lars. When Lars is revealed to be Fry's older (and this time wiser) duplicate, Fry should probably recognize that he could woo Leela if only he'd successfully reign in his adolescent nature. But it being Fry, he fails to take the lesson to heart, and quickly moves on to another girl.

How to Travel Through Time

The Time Ships by Stephen Baxter: In Baxter's sequel to H.G. Wells The Time Machine, we learn that the Time Traveller didn't build his device completely unaided. A mysterious benefactor gave the Traveller a sample of a radioactive substance to study, a substance that ultimately makes time travel possible. Of course, like all mysterious strangers in time travel stories, the Time Traveller's benefactor is, in fact, an older version of himself.

How to Save the World

Heroes "Five Years Gone:" One of the great things about the power to travel through time is that if you get that whole "save the world" business wrong the first time, you can just keep trying. And Hiro Nakamura has the added benefit of traveling through time to change events himself, and leaving instructions for his much less bad-ass past self.

Doctor Who "Time Crash:" The Doctor meets up with himself a great deal, if for no other reason than two or three or five Doctors are better than one. But sometimes it's just to ensure a little predestination paradox magic. The Fifth Doctor watches the Tenth Doctor create an artificial supernova that cancels out a giant hole in fabric of reality. Naturally, the Tenth Doctor only knows how to do this because he watched himself do it when he was the Fifth Doctor.

Doctor Who "The Parting of the Ways:" Rose Tyler gets her own predestination paradox going when she looks into the heart of the TARDIS. The TARDIS gives her the power to transcend time and space, letting her leave the message "Bad Wolf" to herself in the past that ultimately lead Rose and the Doctor back to this time and place.

Teen Titans "Titans Tomorrow:" When the Teen Titans travel to the future, they're eager to see what they're like as adult superheroes. But the future is unexpectedly bleak, with many of the Titans turned to violence and destruction, tearing the United States in two and turning the Western half into a police state. Fortunately, the Titans are able to learn from their future selves what set these events in motion, and are able to prevent their dystopic future.

Babylon 5: To add another wrinkle in the predestination paradox, Jeffrey Sinclair finds that his entire life is being guided by his future self from the past. Sinclair eventually learns that he is the great Minbari historical figure Valen, and Sinclair must eventually travel back in time, become Valen, and write the prophesies that will guide Sinclair's life in the future. Fate, or proof that his talents transcend time and space?

Stephen Baxter's Manifold Trilogy

In these three novels and a few related short stories, Baxter explores possible solutions to the Fermi paradox. His first Manifold novel, Time, operates under the conceit that we really are the only ones around, despite high-probability estimates to the contrary. Space, Baxter's second Manifold novel, asserts that there have been a multitude of other civilizations, but various cosmic disasters destroy them before they are able to make connections. The third novel in the series, Origins, posits that intelligent life is actually separated into parallel universes, so that it is impossible for two different civilizations to contact each other. Baxter's Manifold short stories, which are collected in the book Phase Space, explore these and other possible answers to this perplexing question.

Arthur C. Clarke's 2001: A Space Odyssey

Everybody knows this famous novel of space exploration and the pitfalls of advanced technology. In this story, Clarke postulates that intelligent life does exist independent of our planet and our species — but we're not smart enough to understand their messages. The limited awareness of humans is probably the most plausible explanation for the Fermi paradox, but it's also quite a depressing one.

Terry Bisson's "They're Made Out of Meat"

This Nebula-nominated short story, which Bisson has made available online, is at once hilarious and chilling, an all-dialogue portrayal of intelligent extraterrestrial beings who decide that we're far too primitive to even contact. "What is there to say? 'Hello, meat. How's it going?'" That's one solution of the Fermi paradox — the aliens are here, but they're too snotty to pay us any mind.

David Brin's Uplift series

Brin's Uplift stories, beginning with the 1980 novel Sundiver, contain another assertion that humanity is vastly simple compared to other lifeforms. In this universe, civilizations are not permitted to interact with other intelligent life until they have been "Uplifted" — and that only happens when a vast galactic society decides that they are not only sentient, but sapient. Since every other species in Brin's novels has been found by a far more advanced civilization, genetically modified for thousands of years, and then uplifted, the evolution of the human race seems something of a mystery. Our unique independent development would explain our puzzlement with the Fermi paradox.

Stanislaw Lem's Solaris

In Lem's novel, which has twice been translated to feature films, he explores the idea that alien intelligence operates on a totally different level from our own. Humans who venture to the planet Solaris do discover an intelligent lifeform there, but they are incapable of communicating with it in any way that they understand. Instead, the organism manipulates their emotions and their thoughts without revealing its own, and in the end the planetary researchers are left confused and half-insane. Though this is, again, a depressing idea, it still leaves us with the hope that our society might one day advance enough to commune with others and move forward.

I'm sure you have even more recommendations for Fermi paradox stories, and I urge you to share them with io9 in the comments — but do it quickly. As panelist Michael A. Burstein pointed out, "Wouldn't it be funny if we got a signal from aliens tomorrow and this whole conversation was moot?"

Note: I disqualified stories where the backstory goes back billions of years. If an entity turns up that's already billions of years old, big whoop. We have to follow the story across those millions or billions of years, or it's not worth anything. Sorry, Rama!

House of Suns by Alistair Reynolds. This forthcoming novel is a quasi-sequel to the novella Thousandth Night, which takes place in the year one million A.D. (and appears in the anthology One Million A.D., edited by Gardner Dozois.) The sequel takes place around 6.4 million A.D., meaning the whole loosely-connected story spans about five million years. A galactic civilization challenged by the impossibility of faster-than-light travel decides to move all the inhabited star systems closer together to allow for easier trading and contact. Meanwhile, a post-human family specializes in reclaiming ancient ringworlds.
Time span: five million years, sort of.

Evolution by Steven Baxter. This novel in stories follows human evolution, from tiny mammals 65 million years ago to our posthuman and non-biological descendants 500 million years from now, when Earth is uninhabitable.
Time span: 565 million years.

The First And Last Men by Olaf Stapledon. This 1930 novel describes 18 stages of human evolution, across two billion years, ending with humans living on Neptune and being destroyed by a supernova. Lots and lots of funny-shaped heads, until the humans move to Neptune and become dwarves due to the high gravity.
Time span: two billion years.

The Lensmen novels by E.E. "Doc" Smith. The saga begins over two billion years ago, when the Arisians first realize they need to defend our universe against the marauding Eddorians, and start their breeding program on Earth. This leads to the "Lensmen" two billion years later, and the formation of the Galactic Patrol. It takes hundreds of years for the most deserving of the Lensmen to be born, in the endgame of the Arisians' eugenics program.
Time span: Over two billion years.

Spin by Robert Charles Wilson. This Hugo-winning novel cheats a bit on the aeons front, because alien machines put an artificial membrane around the Earth that blocks out the stars and causes time to pass much more slowly on Earth. One Earth year equals 100 million years for the outside world, so that four billion years pass within a single human generation. It turns out the alien machines put the membrane there because Earth's unsustainable development threatened its destruction.
Time span: four billion years.

The Xeelee sequence by Stephen Baxter. It stars, more or less, in the year 3000 when humans "open up" the solar system with wormhole technology. One of our heroes, Michael Poole, is born in 3621. Humans get embroiled in a long-running war with the alien Xeelee, and human planets are conquered by the Squeem and later by the time-traveling Qax. The Xeelee war begins in earnest in the year 100,000. The Xeelee defeat humanity in the year 1 billion, and then the story jumps forward to the distant future, with humans in the generation ship Great Northern on a five million year voyage, while an artificial intelligence named Lieserl explores the center of a sun. Humans learn how to leave our universe just in time to escape its destruction. The book Vacuum Diagrams has a Xeelee sequence chronology.
Time span: at least 10 billion years, probably more like 100 billion.

Macrolife by George Zebrowski. The Bulero family creates the super-strong (but highly explosive) material Bulerite which allowed humans to conquer space starting in 2021. And then in the year 3000, humans start merging with each other, and with cybernetic consciousnesses, into a kind of Borg-esque collective, which treats individual humans as cells in a body. And then finally in the year one hundred billion, one member of the Bulero family is "re-individualized" from the Macrolife collective to help figure out how to survive the end of the universe. He discovers some Macrolife survivors from previous universes.
Time span: one hundred billion years. In your face, Robert Charles Wilson!

Star Maker by Olaf Stapledon. Perhaps realizing that he was never going to win this contest with a paltry two billion years, Stapledon went back to the drawing board and came up with this 1937 story about a disembodied consciousness who leaves Earth and roams time and space. The story includes the thoughts of sentient clouds in the early universe, and roams all the way up to the heat-death of the cosmos. At one point, the main character meets the "Star Maker," the dispassionate creator of the universe.
Time span: billions and billions of years.

OK, I'm sure I missed some vital and awesome time-spanning storylines. What did I miss?