The National Archives and Records Administration recently marked the 45th anniversary of a devastating fire at the National Personnel Records Center (NPRC) in St. Louis, Missouri, that destroyed approximately 16–18 million Official Military Personnel Files (OMPF) documenting the service history of former military personnel discharged from 1912 to 1964.
Shortly after midnight on July 12, 1973, a fire was reported at the NPRC’s military personnel records building in St. Louis, Missouri. The fire burned out of control for 22 hours and it took two days before firefighters were able to re-enter the building. Due to the extensive damage, investigators were never able to determine the source of the fire.
The fire destroyed paper records in 1973 which sadly is before the capability of making digital copies for backup purposes existed.
You can read more in an article by Kerri Lawrence in the National Archives News at https://www.archives.gov/news/articles/archives-recalls-fire.