Cairo, Egypt

Tourist photos often depict Egypt's pyramids presiding over a spacious desert. In fact, they are crowded by one of the world's oldest metropolises.

People have lived in Cairo for some 6,000 years, but explosive growth is relatively modern. Today the city is as vibrant as ever but struggles to accommodate a rapidly swelling population.

With more than 11 million residents, Cairo is currently the only megacity in Africa, a new UN report says, but the urban center will not hold that little for long.

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