Lockheed Martin's Skunk Works has confirmed that it is developing the SR-72 spy plane. The successor to the SR-71 Blackbird, which was capable of Mach 3.5, the SR-72 will be a hypersonic unmanned aircraft capable of Mach 6, or just over 4,500 mph. At hypersonic speeds, the SR-72 will be able to traverse any continent in around an hour -- meaning the US military can strike or surveil any location on Earth in about an hour.