It's difficult to describe Nairobi's Mathare slum, because there is nothing to which it can be compared where you and I live. But try to imagine 500,000 people crammed into a hillside community of tens of thousands of tiny shacks clad with rusted pieces of corrugated metal. They are akin to structures where you might store old tires that you would never plan on using again. Imagine just a few water pumps and public outhouses to serve all those people. Imagine the stench of open sewage and rotting garbage permeating the air. That's Mathare.