The worldwide shipping industry—the fleet of giant ships carrying more than half the goods moved around the world each year—owns more than 17 million shipping containers. About 20 percent of those containers are empty, and they’re not always in the right place to be loaded with goods for the next shipment. Moving one of these behemoths—whether on a truck, by rail, or by sea—demands a lot of fuel. Simply loading one on or off a ship or truck takes energy and time, leaving ships idling in port and trucks idling in line—and that burns through even more fuel.