The Dharavi squatter settlement in Mumbai is often described as the biggest slum in Asia. It sits between two rail lines in the northern part of the city, on a creek that once sustained a thriving fishery. The creek is now a sump of sewage and industrial waste, and the air above Dharavi is foul.

By one estimate, the slum is home to 10,000 small factories, almost all of them illegal and unregulated. The factories provide sustenance of a sort to the million or so people who are thought to live in Dharavi, which at 432 acres is barely half the size of New York City's Central Park. There is no discernible garbage pickup, and only one toilet for every 1,440 people. It is a vision of urban hell.

It is also one of India's newest tourist attractions. Since January of last year, a young British entrepreneur, Christopher Way, and his Indian business partner, Krishna Poojari, have been selling walking tours of Dharavi as if it were Jerusalem's walled city or the byways of Dickens' London. There seems to be a market for this sort of thing: almost every day during the recent December holidays, small groups of foreign travelers, accompanied by Poojari or another guide, tramped through Dharavi's fetid alleys in a stoic quest for...What? Enlightenment? Authenticity? The three-hour excursions are slated for mention in a forthcoming Lonely Planet guide, and they cost about $6.75 a head—more if you want to go to Dharavi by air-conditioned car.

Poverty tourism—sometimes known as "poorism"—did not originate in Mumbai (formerly Bombay). For years, tour operators have been escorting foreign visitors through Rio de Janeiro's infamous favelas, with their drug gangs and ocean views, and the vast townships outside Cape Town and Johannesburg, where tourists are invited to mix with South Africans at one of the illicit beer halls known as shebeens. A nonprofit group in New Delhi charges tourists for guided walks through the railway station, to raise money for the street children who haunt its platforms.

But the Dharavi tours have been especially controversial. In a lengthy report last September, the Indian English-language Times Now television channel attacked them as an exercise in voyeurism and a sleazy bid to "cash in on the Ôpoor-India' image." That report was followed by a panel discussion in which the moderator all but accused Poojari of crimes against humanity. "If you were living in Dharavi, in that slum, would you like a foreign tourist coming and walking all over you?" he sputtered. "This kind of slum tourism, it is a clear invasion of somebody's privacy....You are treating humans like animals." A tourism official on the panel called the tour operators "parasites [who] need to be investigated and put behind bars," and a state lawmaker has threatened to shut them down.

The critics, it seemed, had claimed the moral high ground. But could they hold it?

One sunny morning this past December, I met Christopher Way at Leopold's Café, a popular backpackers' hangout in Mumbai's bustling Colaba district. At 31, he is boyish and bespectacled, with a thatch of tousled brown hair and a thoughtful, unassuming manner. Over glasses of freshly squeezed mango juice, he told me that he grew up near Birmingham, England, and after graduation from Birmingham University, set off on a path to become a chartered accountant. But Way was afflicted with chronic wanderlust. In 2002, he visited Mumbai and liked the city so much that he stayed five months, volunteering as an English teacher and cricket coach in a local elementary school. He subsequently took an extended holiday in Rio, where he signed up for one of the favela tours. Although frustrated by the guide's lack of knowledge about the shantytown, Way says he found the experience fascinating. It occurred to him that he might be able to do something similar in Mumbai.

As many as half of that city's 18 million or so residents live in squatter settlements, so there was no shortage of potential venues. But Dharavi, as the largest and most established of Mumbai's slums, was the obvious choice. Way's idea was to showcase the settlement's economic underpinnings in a way that would challenge stereotypes about the poor. "We're trying to dispel the myth that people there sit around doing nothing, that they're criminals," he said after we had walked across the street to his office, a grubby, windowless space barely big enough for his desk and laptop computer. "We show it for what it is—a place where people are working hard, struggling to make a living and doing it in an honest way."

To smooth things out with local bureaucrats and Dharavi residents, Way needed an Indian partner, and he found one in Poojari, now 26, a farmer's son who had migrated to Mumbai as an unaccompanied 12 year old and put himself through night school by working in an office cafeteria. The two men formed a company, Reality Tours & Travel, and bought a pair of air-conditioned SUVs. Way bankrolled the venture with income from rental properties he owns in England. Besides the Dharavi tours—which can be combined with visits to Mumbai's red-light district and Dhobi Ghat, a vast open-air laundry—the company offers sightseeing of a more conventional nature, along with hotel bookings and airport transportation. Way has pledged that once the company starts making a profit, it will donate 80 percent of its slum-tour earnings to a charitable group that works in Dharavi. "I didn't want to make money from the slum tours," he says. "It wouldn't have felt right."

Except on its Web site (Leopold's Café—"See Dharavi (the biggest slum in Asia)"—the company does not advertise the slum excursions. But as word has spread over the Internet and by other means, business has grown steadily, drawing visitors from around the world.

Late one morning I met Poojari at the Churchgate railway station, where we hopped on a dilapidated commuter train for the 25-minute ride to Dharavi. Waiting for us there was tourist Jeff Ellingson, a 29-year-old technology professional from Seattle. Before we got started, Poojari explained that the company has a no-photography policy, to keep the tours from becoming too intrusive. (For the same reason, each group is limited to five people.) Then we took a pedestrian bridge over the railroad tracks.

Dharavi stretched before us like a vast junkyard, a hodgepodge of brick and concrete tenements roofed with corrugated metal sheets that gleamed dully in the sunshine. Poojari gave us a moment to take it all in. "We'll show you the positive side of a slum," he declared.

In the face of such squalor, his words seemed jarring. But Dharavi's industriousness is well documented. Its businesses manufacture a variety of products—plastics, pottery, bluejeans, leather goods—and generate an estimated $665 million in annual revenue. In other words, Dharavi is not just a slum, it is also a node on the global economy.

Dharavi's industries are arranged geographically, like medieval guilds, and the first alley we visited belonged to recyclers. In one small "godown" (as warehouses are known on the subcontinent), men were disassembling old computer keyboards. In another, men smeared from head to toe in blue ink stripped the casings from used ballpoint pens so they could be melted down and recycled. A few doors down, workers used heavy chains to knock the residue from steel drums that had once contained polyester resin. Poojari told us that some of Dharavi's empty plastic bottles come from as far away as the United Kingdom. "People from a rich family, when they drink from a plastic bottle, they don't know what happens to it afterwards," he said. "Here, you see."

Few of the recyclers wore gloves or other protective gear, despite exposure to solvents and other chemicals that caused my eyes and throat to burn after just a few minutes. The working conditions were typical of Dharavi's unregulated businesses. Some of the worst were in the foundries. From the door of one dark, unventilated space, I watched a heavyset worker dressed in a sarong ladle molten steel into a belt-buckle mold that he held between his feet. His bare feet. After cracking open the mold to reveal the glowing red buckle in its bed of sand, he glanced up, and for a moment our eyes met. His face was wooden, expressionless. I mumbled thanks and moved on.

Not for the first time on the tour, I felt like an interloper, and I wondered how the slum workers and their families felt about white-skinned strangers who showed up to gawk from the threshold. For Dharavi was undeniably grim. As we neared its center, the alleys narrowed and cantilevered balconies closed out the sun, casting everything in a permanent gloom. Children played next to gutters that flowed with human waste, and hollow-eyed men bent nearly double under the weight of burlap-covered loads. But if the people of Dharavi resented us, they kept it to themselves. Some even seemed happy to take part in our education. "Here, everybody is working," a man said genially, and in perfect English, as we paused outside the yogurt-cup recycling operation where he sat sipping tea with the owner.

The welcoming reception probably had something to do with the tour operators, who have cultivated good relations with the slum workers as well as local police. There are, moreover, certain rules. From the door of a one-room garment factory, I spotted a boy who looked to be no more than 8 sitting with other workers at a long table, where he was embroidering fabric with fine gold thread. I nudged my guide: "Ask him how old he is." Poojari shook his head no. Pointed questions were not part of his compact with the slum dwellers.

As it happens, Ellingson and I did not see many child laborers in Dharavi, perhaps because of laws limiting employment of children under 14 or, more likely—as Way suggested later—because they were sequestered out of view. We did see several schools, however, and plenty of kids in uniforms. "By plane you are coming?" one boy asked in English, before declaring, with evident pride, "I'm studying in 8th standard."

Blighted though it was, Dharavi had the feeling of an established community. Signs in Hindi advertised the services of doctors and dentists. An outdoor barber administered a shave with a folding razor. A laundryman stood against an alley wall, pressing clothes with an ancient-looking iron. At a small factory where recycled plastic was melted down and turned into tiny pellets for use by toymakers, the owner, who was in his late 20s, told us that his father had started the business three decades ago. Like many of the slum dwellers, the factory owner was a Muslim, although Dharavi is nothing if not diverse. Its residents come from all over India, and many have lived there for a generation or more. Poojari said that one of the slum neighborhoods is dominated by the descendants of potters from Gujarat state who settled in Dharavi in 1933. When we visited the potters' district in the early afternoon, we were puzzled to find few signs of life, other than smoking kilns and an old man napping on a rope cot. It turned out that most of the potters and their families had taken the afternoon off to attend a wedding.

Ellingson drew a comparison with Palestinian communities he had toured in the West Bank. They were "a lot wealthier, but it's like society has broken down," he said, adding that in Dharavi, "it feels like something is functioning." I had to agree.

For one thing, almost no one asked for money, or even tried to sell me anything. Only once was I approached for a handout, by an elderly woman. That was a big change from Colaba, the main tourist district, where it is difficult to walk more than a few steps without being accosted by a beggar—usually a young woman with an infant on her hip—or a peddler hawking laminated maps. Perhaps people in Dharavi were simply too busy. And some of them clearly had rupees in their pockets. Besides the food stalls and handcarts piled high with okra and squash, there were video parlors showing Bollywood hits, several bars and, on one thoroughfare, a spiffy-looking electronics store plastered with Sony decals.

"They're happy to be here," Poojari said as we paused outside a small factory where women were stitching bluejeans. "They don't want to move out of Dharavi." I don't know about "happy," but on the latter point, Poojari was probably correct.

Because of its prime location—on rail lines near the heart of one of the world's most crowded cities—Dharavi sits on valuable real estate, and its residents are not about to give up their stake. The slum is the focus of a looming showdown with municipal authorities and developers who want to turn it into office towers, luxury apartments and shopping centers. Families that can prove they have lived in Dharavi since 1995 would be entitled to a free apartment in the same area, but the new dwellings would be so small—just 225 square feet—that many prefer to stay where they are. Nor is it clear what would happen to the thousands of businesses that provide Dharavi residents with jobs.

We finished the tour on the side of a busy four-lane road, where the festive sounds of a Hindu wedding ceremony—apparently the one the potters had gone to—spilled from a gaudy tent. We paused to peek inside, and I spotted the groom sitting awkwardly beneath an enormous gold turban. No one gave us a second glance, and I had to wonder about the motives of those in the Indian media and elsewhere who claimed on behalf of the Dharavi residents to be offended by the tours. Surely their ire could have been better targeted at the municipal authorities who had failed to provide the community with basic sanitation. I wondered whether the critics weren't simply embarrassed by the slum's glaring poverty—an image at odds with the country's efforts to rebrand itself as a big software park. In any case, it seemed to me that the purpose of the tour was not to generate pity, but understanding. That's not to say that it made me an expert—I was only there a few hours, after all. Were the people I saw in Dharavi the victims of globalization, or its beneficiaries? I still don't know. But at least the question had been raised in my mind.

John Lancaster recently completed a four-year assignment as the South Asia correspondent for the Washington Post.