Imagine Lagos, Nigeria, a city of 22 million. What was once a small coastal town just a few decades ago has exploded into a sprawling megacity spanning 452 square miles. Its rapid growth has stretched the city’s services impossibly thin: Less than 10 percent of people live in homes with sewer connections; less than 20 percent have access to tap water. Many houses are in slums and informal settlements at the city’s periphery. Now picture Lagos twice as big.