The effect was known from the earliest days of nuclear testing when radar systems were used to track the nuclear mushroom clouds at very long distances . Its extended effects when exploded outside the atmosphere were first noticed in 1958 as part of the Hardtack and Argus nuclear tests , which caused widespread radio interference extending over thousands of kilometers . The effect was so disconcerting that both the Soviets and US broke the informal testing moratorium that had been in place since late 1958 to run series of tests to gather further information on the various high-altitude effects like blackout and electromagnetic pulse ( EMP )
Hardtack
Operation Hardtack I
Nuclear blackout