It used to be the stuff of 2000 AD, the comic that introduced the world to Judge Dredd and two vast crime-filled cities, Mega City One and East Meg One.
In its dystopian vision, the first mega city around New York began construction in 2030, intended to house three to four million people.
In a sign of how quickly future dystopias age, the new Times Atlas of the World lists the growing club of real mega cities, all of them with predicted populations of more than 10 million - not by 2030, but by 2005.
According to these estimates, Tokyo - the world's largest city - will hit nearly 27m. São Paolo in Brazil will reach just under 20m and Mexico City 19m. Sixteen other cities are expected to exceed the 10m mark, including Bombay (Mumbai) 18m, and Dhaka in Bangladesh, 15m.
Two cities in Africa are expected to go mega - Lagos in Nigeria and Cairo in Egypt. According to the atlas - the 11th edition since it was first published in 1895 - the phenomenon is a mark of a global population in the grips of rapid urbanization, where close to 50 per cent of the population now lives in cities.
Indeed, the latest estimates predict that urban dwellers will outnumber the rural population for the first time by 2007.
And Tokyo is leading the way. A Landsat 7 image of the city, included in the atlas, shows the city's growth, a spreading gray cancer whose spiraling tendrils can be seen sucking in neighboring cities and towns and even reclaimed sea.
The rise of the world's mega cities is one of the most marked trends noted by the atlas in recent decades. In 1950 New York City was the only one of the world's cities with more than 10m inhabitants. By 1975 that number had grown to five. By 2015 it is estimated there will be 21.
It has been a process driven largely by Asia - the continent boasting 10 mega cities by 2000, while North America had managed two (New York City and Los Angeles).
But the mega cities are not the only major human impact noted by the atlas. There has also been a catastrophic impact on the environment. The atlas's authors estimate that 90,000 square kilometers (35,500 sq miles) of forest are being lost each year, the equivalent, since the last edition of the atlas in 1999, of an area the size of the British Isles.
But the greatest impact has come through global warming, with successive editions of the atlas showing shrinking ice fields and evaporating lakes.
It reveals the rapid retreat of the Aral Sea in Kazakhstan and Uzbekistan, once the world's fourth largest lake and now the tenth. Since the 1967 edition of the atlas it has shrunk by 39,994 sq km (15,800 sq m).
Since the 1975 edition, the surface of the Dead Sea has dropped by a massive 17 meters.
It is the availability of new digital satellite technology that has made the changes so shockingly apparent.
The atlas's chief cartographer, Sheena Barclay, said: 'We are seeing things that you would not have seen 10 or even 15 years ago, changes that we can see by overlaying versions of our satellite images. And we are seeing a lot of concerning things.'
Perhaps the most compelling evidence of global climate change has come not between editions of the atlas but during the preparation of the present volume when the cartographers had to redraw the coastline of Antarctica after the Larsen ice shelf, which is the size of Luxembourg, disintegrated last year.
© Guardian Newspapers Limited 2003