Reaching bottom after a 2-hour-and-36-minute descent, the National Geographic explorer and filmmaker typed out welcome words for the cheering support crew waiting at the surface: "All systems OK." Folded into a sub cockpit as cramped as any Apollo capsule, the National Geographic explorer and frilmmaker is now investigating a seascape more alien to humans than the moon. Cameron is only the third person to reach this Pacific Ocean valley southwest of Guam (map)—and the only one to do so solo. Hovering...