The War of the Worlds is an episode of the American radio drama anthology series The Mercury Theatre on the Air. It was performed as a Halloween episode of the series on October 30, 1938, and aired over the Columbia Broadcasting System radio network. Directed and narrated by actor and future filmmaker Orson Welles, the episode was an adaptation of H. G. Wells' novel The War of the Worlds (1898).
The first two thirds of the 60-minute broadcast were presented as a series of simulated news bulletins, which suggested to many listeners that an actual alien invasion by Martians was currently in progress. Compounding the issue was the fact that, the Mercury Theatre on the Air was a sustaining show (it ran without commercial breaks), adding to the program's realism. Although there were sensationalist accounts in the press about a supposed panic in response to the broadcast, the precise extent of listener response has been debated.
More information on the Wikipedia page