Credit: (AP Photo/Jon Gambrell)

Violent crime rates in cities have declined significantly since the mid-1990s. We examine the reasons behind this drop, and the influence it has had on city life.

Then, concrete buildings are the foundation of the modern world. But they eat up a resource that’s becoming increasingly difficult to come by: sand.

Finally, for centuries, species have mutated to adapt to urban habitats. We investigate the wily ways that they continue to evolve in cities.