We explore the hidden role of cobalt in the A.I. revolution. The element is a key ingredient of lithium-ion batteries, which power everything from cell phones to laptops to electric vehicles. But the way cobalt is mined is troubling. New Yorker journalist Nicolas Niarchos takes us to the Democratic Republic of Congo, which accounts for more than 70% of the world's cobalt supply. There, he exposes child labor in the supply chain and reveals a new frontier of competition with China.