Imagine Lagos, Nigeria, a city of 22 million people. What was once a small coastal town just a few decades ago has exploded into a sprawling megacity stretching over more than 1,170 square kilometers. This rapid growth has strained municipal services to their limit: less than 10% of people live in homes connected to sewer networks, and less than 20% have access to running water. Many homes are in slums or informal settlements on the city’s periphery.

Now imagine Lagos twice as large.