THE MEGACITY.

The Third Mainland Bridge is a looping ribbon of concrete that connects Lagos Island to the continent of Africa. It was built in the nineteen-seventies, part of a vast network of bridges, cloverleafs, and expressways intended to transform the districts and islands of this Nigerian city--then comprising three million people--into an efficient modern metropolis. As the bridge snakes over sunken piers just above the waters of Lagos Lagoon, it passes a floating slum: thousands of wooden houses, perched on stilts a few feet above their own bobbing refuse, with rust-colored iron roofs wreathed in the haze from thousands of cooking fires. Fishermen and market women paddle dugout canoes on water as black and viscous as an oil slick. The bridge then passes the sawmill district, where rain-forest logs--sent across from the far shore, thirty miles to the east--form a floating mass by the piers. Smoldering hills of sawdust landfill send white smoke across the bridge, which mixes with diesel exhaust from the traffic. Beyond the sawmills, the old waterfront markets, the fishermen's shanties, the blackened facades of high-rise housing projects, and the half-abandoned skyscrapers of downtown Lagos Island loom under a low, dirty sky. Around the city, garbage dumps steam with the combustion of natural gases, and auto yards glow with fires from fuel spills. All of Lagos seems to be burning.

The bridge descends into Lagos Island and a pandemonium of venders' stalls crammed with spare parts, locks, hard hats, chains, screws, charcoal, detergent, and DVDs. On a recent afternoon, car horns, shouting voices, and radio music mingled with the snarling engines of motorcycle taxis stalled in traffic and the roar of an air compressor in an oily tire-repair yard. Two months earlier, a huge cast-iron water main suspended beneath the bridge had broken free of its rusted clip, crushing a vacant scrap market below and cutting off clean water from tens of thousands of the fifteen million people who now live in Lagos.

In the absence of piped water, wealthier residents of the waterfront slum at the end of the bridge, called Isale Eko, pay private contractors to sink boreholes sixty feet deep. All day and night, residents line up at the boreholes to pay five cents and fill their plastic buckets with contaminated water, which some of them drink anyway. Isale Eko is the oldest and densest part of Lagos Island. Every square foot is claimed by someone--for selling, for washing, even for sleeping--and there is almost no privacy. Many residents sleep outdoors. A young man sitting in an alley pointed to some concrete ledges three feet above a gutter. "These are beds," he said.

In the newer slums on the mainland, such as Mushin, rectangular concrete-block houses squeeze seven or eight people into a single, mosquito-infested room--in bunks or on the floor--along a narrow corridor of opposing chambers. This arrangement is known as "face me I face you." One compound can contain eighty people. In Mushin, Muslim Hausas from the north of Nigeria coexist uneasily with mostly Christian Yorubas from the south. Armed gangs represent the interests of both groups. On the night of Febru-ary 2, 2002, a witness told me, a Hausa youth saw a Yoruba youth squatting over a gutter on the street and demanded, "Why are you shitting there?" In a city where only 0.4 per cent of the inhabitants have a toilet connected to a sewer system, it was more of a provocation than a serious question. The incident that night led to a brawl. Almost immediately, the surrounding compounds emptied out, and the streets filled with Yorubas and Hausas armed with machetes and guns. The fighting lasted four days and was ended only by the military occupation of Mushin. By then, more than a hundred residents had been killed, thousands had fled the area, and hundreds of houses had burned down.

Newcomers to the city are not greeted with the words "Welcome to Lagos." They are told, "This is Lagos"--an ominous statement of fact. Olisa Izeobi, a worker in one of the sawmills along the lagoon, said, "We understand this as 'Nobody will care for you, and you have to struggle to survive.' " It is the singular truth awaiting the six hundred thousand people who pour into Lagos from West Africa every year. Their lungs will burn with smoke and exhaust; their eyes will sting; their skin will turn charcoal gray. And hardly any of them will ever leave.

Immigrants come to Lagos with the thinnest margin of support, dependent on a local relative or contact whose assistance usually lasts less than twenty-four hours. A girl from the Ibo country, in the southeast, said that she had been told by a woman in her home town that she would get restaurant work in Lagos. Upon arrival, she discovered that she owed the woman more than two hundred dollars for transport and that the restaurant job didn't exist. The girl, her hair combed straight back and her soft face fixed in a faraway stare, told me that she was eighteen, but she looked fifteen. She is now a prostitute in a small hotel called Happiness. Working seven nights a week, with each customer spending three and a half dollars and staying five minutes, she had paid off her debt after seven months. She has no friends except the other girls in the hotel. In her room, on the third floor, the words "I am covered by the blood of Jesus. Amen" are chalked on a wall three times.

A woman named Safrat Yinusa left behind her husband and two of her children in Ilorin, north of the city, and found work in one of Lagos's huge markets as a porter, carrying loads of produce on her head. She was nursing a baby boy, whom she carried as she worked. She paid twenty cents a night for sleeping space on the floor of a room with forty other women porters. In two months, she had saved less than four dollars. Considering that the price of rice in Lagos is thirty-three cents per pound, it is hard to understand how people like Yinusa stay alive. The paradox has been called the "wage puzzle."

When Michael Chinedu, an Ibo, arrived in Lagos, he knew no one. On his first day, he saw a man smoking marijuana--in Lagos, it's called India hemp--and, being a smoker as well, introduced himself. On this slim connection, Chinedu asked the man if he knew of any jobs, and he was taken to the sawmill, where he began at once, working long days amid the scream of the ripsaw and burning clouds of sawdust, sleeping outside at night on a stack of hardwood planks. After three months, he had saved enough for a room. "If you sit down, you will die of hunger," he said.

The hustle never stops in Lagos. Informal transactions make up at least sixty per cent of economic activity; at stoplights and on highways, crowds of boys as young as eight hawk everything from cell phones to fire extinguishers. Begging is rare. In many African cities, there is an oppressive atmosphere of people lying about in the middle of the day, of idleness sinking into despair. In Lagos, everyone is a striver. I once saw a woman navigating across several lanes of traffic with her small boy in tow, and the expression on her face was one I came to think of as typically Lagosian: a look hard, closed, and unsmiling, yet quick and shrewd, taking in everything, ready to ward off an obstacle or seize a chance.

In 1950, fewer than three hundred thousand people lived in Lagos. In the second half of the twentieth century, the city grew at a rate of more than six per cent annually. It is currently the sixth-largest city in the world, and it is growing faster than any of the world's other megacities (the term used by the United Nations Center for Human Settlements for "urban agglomerations" with more than ten million people). By 2015, it is projected, Lagos will rank third, behind Tokyo and Bombay, with twenty-three million inhabitants.

When I first went to Lagos, in 1983, it already had a fearsome reputation among Westerners and Africans alike. Many potential visitors were kept away simply by the prospect of getting through the airport, with its official shakedowns and swarming touts. Once you made it into the city, a gantlet of armed robbers, con men, corrupt policemen, and homi-cidal bus drivers awaited you.

Recently, Lagos has begun to acquire a new image. In the early years of the twenty-first century, the Third World's megacities have become the focus of intense scholarly interest, in books such as Mike Davis's "Planet of Slums," Suketu Mehta's "Maximum City," and Robert Neuwirth's "Shadow Cities." Neuwirth,…