TOP 5

As people continue to migrate away from rural areas and into cities, cities that are growing to devour the land around it, the numbers of people living in slums, shanty towns and informal settlements are skyrocketing.
Currently, there are 200,000 of these communities across the world, according to the United Nation's Special Rapporteur on Adequate Housing, most of them in and around cities, and that number is growing exponentially. Even before the economic crisis of 2008, about one third of all city dwellers live in slums, slums which will grow in size by one billion more people within the next twenty years.
While 90 percent of the world's informal settlements are in developing nations -- such as India and Brazil -- they are a worldwide phenomenon and are in European capital and largest American cities. Here are the five largest slums in the world: 

    1. Neza-Chalco-Itza: One of Mexico City's many barrios, Neza-Chalco-Itza is the largest slum in the world with roughly four million people. Slums in Mexico City began growing in the early 1900s when the railroad brought new industry -- as well as new industrial zones -- to the city.
As with many slums, most people in Neza-Chalco-Itza live illegally on unauthorized land. However, unlike in most areas, some of Mexico City's slum dwellers also live in vecindades, former mansions abandoned by wealthy families that have been converted into low-income rental apartments. About 10 percent of all the residents of the Mexican capital live in these buildings.  

    2. Orangi Town:  Technically only 10 years old, the township in Karachi, Pakistan is home to 1.5 million people and still growing. Orangi, with 22 square miles of space, is significantly less dense than most urban slums and also more structured. There are 13 official neighborhoods, each with its own council, which has allowed the township to build its own sewer system. Additionally, as only of 18 districts of Karachi, Orangi has government representation, albeit in the lowest tier of the government.

    3. Dharavi: After Orangi, the largest slum in Asia is Dharavi in Mumbai. About one million people reside on just one square mile of space that was formerly a mangrove swamp. People became flocking to the area as the tanning and textile industries boomed and Dharavi's population density is now 11 times higher than that of the city that encompasses it.
A neighborhood smack in the heart of Mumbai, it retains the emotional and historical pull of a subcontinental Harlem -- a square-mile center of all things, geographically, psychologically, spiritually, National Geographic said in a recent article. Its location has also made it hot real estate in Mumbai, a city that epitomizes India's hopes of becoming an economic rival to China.
In 2011, other Mumbai slums might have surpassed Dharavi in total population, but the figures can only be speculated at this point. What that does mean, though, is that a number of Asia's largest shanty towns are all in the same city (Notably, Mumbai is the fifth biggest city in the world).

                                                           Dharavi in Mumbai
 
     4. Khayelitsha: Cape Town, South Africa is a huge, sprawling city that has given rise to a number of new neighborhoods and townships over the past decade. Khayelitsha is now the biggest of these and the community had a population explosion after apartheid ended and blacks rushed into Cape Town for jobs.
As of the last census in 2005, there were a recorded 400,000 people in Khayelitsha, but that figure is likely much higher. The township's population is incredibly young, with 40 percent of its residents under 19 years old and only about seven percent over the age of 50.
      5. Kibera:  A neighborhood in Nairobi, Kenya, the Kibera slum is the second largest in Africa, with anywhere from 200,000 to one million residents. Kibera is often used as the model for the environmental impact of informal settlement. Without state sponsored infrastructure like plumbing, residents resort to using flying toilets, which are just plastic bags that can be tossed onto the street. However, Kibera is also one prototype for urban renewal and slum upgrading projects. For about $300,000, U.N. HABITAT is trying to transform all of Kenya's slums, staring with the construction of roads and other services and an actual mapping of the area.
Rounding out a list of the 10 biggest slums would include communities in Bogota, Colombia; Baghdad, Iraq, Venezuela, and Ghana.
The conditions in slums are surely a seriously problem with global implications, but slums themselves do not have to be a problem for anyone. The global population is booming -- the seven billionth citizen of Earth was born only a month ago -- and people need to live somewhere. In the modern, global economy, urbanization is a trend that is impossible to buck and informal settlements are a necessary part of the process.
There are ways to make these communities more hospitable and less hazardous on an environmental level. Some cities in Colombia are extending public transportation lines out to the slums, which as encouraged economic and urban development.
Additionally, others see the population density of slums as a blessing rather than a curse, and perhaps as the future of cities. New, conceptual low-income housing projects are springing up in the United States that could be a model for the developing world. Single-family stacked houses like the ones currently in Brazil and Mexico could be the prototype for the next generation of cities.