What exactly is an underwater drone?

News broke today that a Chinese warship had seized an underwater drone belonging to the US Navy. The move is sure to escalate tensions around the South China Sea, a contested area where China has increasingly been flexing its muscle. We cover a lot of drones at The Verge, but they are typically small, consumer-facing aircraft that would be very unhappy if you got them wet.
So what is an underwater drone? It’s basically a miniature submarine without any people inside. Because it’s muc