The Ubatuba Fragments
The account
A disc supposedly exploded over the beach and left behind pieces of magnesium too pure to be of Earthly origin.
What investigators found
No one ever identified the witnesses, and later analyses, including some in the United States, found ordinary impurities in the metal.
The story
In September 1957, Ibrahim Sued, a columnist for the newspaper O Globo, received an anonymous letter containing metal fragments. The sender said he was fishing near Ubatuba when he saw a disc plunge toward the sea, climb again and explode, scattering shiny pieces across the beach. Physician and ufologist Olavo Fontes had the material analyzed, and the first report pointed to magnesium of extremely high purity.
The case became a worldwide classic because it would be the only physical sample of a UFO. The problem is that the fisherman was never found, and the story rests entirely on the letter. Later analyses, including one done for the Condon Committee at the University of Colorado, found impurities and concluded the metal was no purer than the magnesium produced at the time. Where the fragments came from remains unanswered.