1. Banjul, Gambia
Cause: Rising sea levels and erosion
As sea levels rise around the world, the small West African nation of Gambia may lose its capital, Banjul, entirely. The city is threatened by a combination of erosion and the rising ocean.

2. Detroit, U.S.A.
Cause: Population flight
Detroit’s population has decreased by a third since 1950 to about 950,000, and it is expected to shrink slowly but steadily until at least 2030–unemployment inside the city is currently more than 10%. (The suburbs around Detroit, meanwhile, are growing.) If trends hold, Detroit will be altered beyond recognition by 2100.

3. Ivanovo, Russia
Cause: Population flight
A center of textile production during the Soviet era, this district capital to the northeast of Moscow has 448,000 inhabitants and falling. Women outnumber men, the birthrate is dropping and the mortality rate has increased since 1990. Educated young people leave for Moscow, as the city has no modern industry of any kind.

4. Mexico City, Mexico
Cause: Lack of drinking water and sinking
Mexico City is sinking, though not into the ocean. The city sits on an aquifer, which is also its main source of drinking water. Each time one of its 20 million inhabitants takes a drink of water, the city sinks a tiny bit more. By some estimates, parts of the city have fallen 9 meters in the last 100 years. Potentially even worse: The aquifer is thought to be running dry. Although Mexico City is currently growing at a rapid clip, a dwindling water supply on sinking ground could quickly reverse the trend.