The Mathare Valley, shown here in an aerial map, is one of the largest and oldest slums in Nairobi, Kenya. Residents are using hand-held GPS devices to map the area, which comprises 13 villages and is home to nearly 200,000 people. Courtesy of Muungano Support Trust and Jason Corburn, UC Berkeley hide caption

itoggle caption Courtesy of Muungano Support Trust and Jason Corburn, UC Berkeley