July 2, 1937
Amelia Earhart and her navigator, Fred Noonan, disappear
Due to neither person nor their plane being found, the disappearance has become a mystery with new evidence and hypotheses coming forward every so often. This includes a forensic anthropology report published in 2018 which claims with 99% certainty the skeletal remains found on Nikumaroro Island in 1940 belong to Earhart. (You can read the full report here.)
The duo was officially declared dead in absentia on January 5, 1939.