It's a mild, sunny winter afternoon in Rio de Janeiro, Brazil, and Rocinha is hopping. People swarm through the narrow streets, vendors hawk their wares, music trickles out of apartment windows in buildings packed closely together.

At first glance, Rocinha is just another crowded neighborhood. Look again. It's actually a slum --one of the 500 to 600 "favelas" that dot the hills in and around Rio. Many of the buildings are badly in need of paint, there's little running water or basic plumbing, and drug lords are said to rule.

Still, many of Rio's poor clamor to live here. Rocinha isn't just another favela, but the "first world of favelas" -- South America's largest and most developed slum. Varying counts put the population anywhere between 150,000 to over 250,000 people.