What object did the U.S. military claim had crashed after it had been suspected of being a UFO, in 1947?

In mid-1947, a United States Air Force balloon crashed at a ranch near Roswell, New Mexico, prompting claims that the crash was of an extraterrestrial spaceship. Following wide initial interest, the military stated that the crash was merely of a conventional weather balloon. Interest subsequently waned until the late 1970s, when ufologists began promoting a variety of increasingly elaborate conspiracy theories, claiming that one or more alien spacecraft had crash-landed, and that the extraterrestrial occupants had been recovered by the military, who then engaged in a cover-up.

In the 1990s, the US military published two reports disclosing the true nature of the crashed aircraft, a surveillance balloon from Project Mogul. Nevertheless, the Roswell incident continues to be of interest in popular media, and conspiracy theories surrounding the event persist. Roswell has been described as "the world's most famous, most exhaustively investigated, and most thoroughly debunked UFO claim".