The mighty Colorado is under threat. From the Rockies' snowy peaks to Mexico, the river is a lifeline for tens of millions of people. We journey along its waters to see places and meet people changed by a drier world.

In the French city of Marseille, there's a war on drugs. The police are cracking down on gangs dealing from estates in the city's north. The dealers say it's the only way to survive. We gain rare access to both sides.