Wilbur and Orville Wright had barely been working on their homemade “flying machine” for a month when they officially filed a patent for their design on March 23, 1903. For years, the patent for the world’s first airplane resided in the National Archives, but in 1980 it appeared to vanish. Now, nearly 40 years later, archivists finally recovered the missing documents in a storage cave in Kansas.

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When a system is dealing with as many documents as the National Archives, it’s not surprising that things can occasionally go missing. The Wright Brothers’ patent was supposed to be kept securely in a special vault in Washington, D.C. alongside other historical treasures and artifacts, Sarah Laskow reports for Atlas Obscura. But when archivists went to retrieve it for a retrospective in 2000, the patent was nowhere to be found.

“If somebody puts something back in the wrong place, it’s essentially lost,” William J. Bosanko, the Chief Operating Officer of the National Archives and Records Administration tells Michael E. Ruane for the Washington Post. “In this case, we didn’t know. We had to ask ourselves, ‘Is it something that could have been stolen?’ ”

For the last 16 years, investigative archivists have scoured archives, filing cabinets and storage rooms across the country for any sign of where the precious patent may have gone. According to the National Archive’s records, the patents were kept in Washington until 1969 when they were sent to a federal records facility in Suitland, Maryland. Some of the files were leant to the Smithsonian for an exhibition, but the records showed they were returned in 1979, Hyacinth Mascarenhas reports for the International Business Times UK. That was the last time that archivists had a record of the patent's whereabouts.

“We had a pull slip from our files saying that the document was returned to the National Archives in 1980,” archivist Chris Abraham tells Ruane. “But . . . that’s where the trail goes cold.”

Abraham had been working for the National Archive’s recovery program for just a few weeks when he volunteered to take a shot at finding the Wright Brothers’ missing files. Having a longstanding interest in the Wright Brothers, Abraham knew that the inventors had several other patents that were not kept in the Archive’s “treasure vault,” but were likely in a special storage facility in a limestone cave in Lenexa, Kansas, Ruane reports.

The National Archives has billions of pieces of paper in its holdings, and there isn’t nearly enough room to keep everything on hand in one place. Many of the Archive’s documents are kept in 18 records centers located throughout the country that are specially-designed for archiving and preserving historic documents. Abraham had a hunch that the Wright Brothers’ patents had been sent to the Lenexa facility, Ruane reports.

Working with Abraham, archivist Bob Beebe spent weeks combing through stacks of boxes. After striking out time and time again, Beebe checked one last box on the morning of March 22. Inside, he found a manila folder labeled “Wright Brothers’ Patents,” containing the long-lost documents that detailed their revolutionary flying machine.

“I was stunned,” investigative archivist Mitchell Yockelson tells Ruane. “If I had to pick one [crucial] document . . . that’s missing, this was it.”

Since its rediscovery nearly 113 years after the day it was first filed, the Wright Brothers’ patent has been returned to Washington, D.C. Luckily, the files won’t be sealed away forever: starting on May 20, some of the documents will be put on public display at the National Archives.