In late 2012, the U.S. Air Force declassified a trove of documents, including records of a secret program to build a flying saucer-type aircraft designed to shoot down Soviet bombers. The ambitious program, called Project 1794, was initiated in the 1950s, and a team of engineers was tasked with building a disc-shape vehicle capable of traveling at supersonic speeds at high altitudes.
The declassified documents reveal plans for the plane to reach a top speed of Mach 4 (four times the speed of sound), and reach an altitude of 100,000 feet (30,480 meters). The project's estimated cost was more than $3 million, which in today's dollars would be more than $26 million.
Project 1794 was canceled in December 1961 after tests suggested the flying saucer design was aerodynamically unstable and would likely be uncontrollable at high speeds (let alone supersonic speeds).
With the black budget topping $81.1 Billion in 2018, can you imagine the sort of technological advancements and scientific knowledge that is kept hidden?