Urbanization usually accompanies social and economic development, but rapid urban growth on today’s scale strains the capacity of local and national governments to provide even the most basic of services such as water, electricity and sewerage. Squatter settlements and over-crowded slums are home to tens of millions, like the favelas that cling to the hillsides of Rio de Janeiro and the tombs used as homes by tens of thousands in Cairo’s "City of the Dead". In some developing countries, notably in Africa, this growth reflects rural crisis rather than urban-based development.