Since World War Two, militaries have relied on radar to detect potentially attacking aircraft. With time and improved technology, it is now used to find everything airborne from supersonic jets and missiles to low-flying helicopters and drones. It is also a tool deployed by aircraft to pick out ships and surfaced submarines on the wide-open oceans. But nothing spurs innovation like military threats. So by the 1970s, engineers had discovered how to build structures that were difficult, if not impossible, to see on radar. In 1978, the U.S. started building stealthy military vehicles that would be invisible to enemy radars.