50 years, but some files are missing. here s the bbc s david shukman. reporter: a few ghostly lights, secret military investigations, fuel for endless curiosity about whether we re alone. the latest official document, some heavily censored, record dozens of apparent sightings, like this strange cloud pictured in sri lanka. they also offer reasonable explanations, including that sometimes it s just a hoax. it might have been the martians have landed. reporter: in 1967 the discovery of six of these in southern england triggered a massive alert but turned out to be a student prank. though, the authorities remained on the lookout. obviously it was taken seriously because not because they believed we were being visited by aliens but they were worried there may be other terrestrial explanations like
released previously classified documents on ufos that date back 50 years, but some files are missing. here s the bbc s david shuckman. reporter: a few ghostly lights, secret military investigations, fuel for endless curiosity about whether we re alone. the latest official document, some heavily censored, record dozens of apparent sightings, like this strange cloud pictured in sri lanka. they also offer reasonable explanations, including that sometimes it s just a hoax. it might have been the martians have landed. reporter: in 1967 the discovery of six of these in southern england triggered a massive alert but turned out to be a student prank. though, the authorities remained on the lookout. obviously it was taken seriously because not because they believed we were being visited by aliens but they were worried there may be other terrestrial explanations like russian spy planes.