Mohammad Ponir Hossain/Reuters
A man in Dhaka, Bangladesh leaps between the crowded train cars.
Urbanization is taking over the world.By the year 2050, 70% of the world will live in cities, according to the Population Reference Bureau.
One of the most striking ways to see this growth is through the lens of population density, or the way statisticians measure congestion.
The densest cities grapple with unique challenges. They fight for resources, like housing and personal space, and they put infrastructure to the test.
Here's just a taste of what life is like.