A cold case team from the U.S. National Archives discovered the missing patent file for the Wright brothers' "flying machine," which has been missing since 1980. However, record keepers of its Washington, D.C., vault only realized it was missing in 2000, afterwards determining that it had been misplaced as far back as 1980. After ramping up its efforts to find the missing paperwork, the team came across the patent in a limestone cave.

The patent file was discovered by Bob Beebe, a volunteer archivist that found it hidden within a 15-foot-high stack of documents located in a special records storage cave in Kansas. After being ready to resign to failure, Beebe looked in one more box and found it - patent No. 821,393.

The file was known to be stored in the National Archives building at one point in time and was then transferred to a federal records center in 1969. In 1979, some parts of the file were lent to the Smithsonian for an exhibit and then returned back to the federal records center.

"We had a pull slip from our files saying that the document was returned to the National Archives in 1980," said archivist Chris Abraham. "But... that's where the trail goes cold."

Abraham believed that the Wrights' patents were in the Lenexa cave, where he asked Beebe to search in an attempt to locate the missing "flying machine" file.

The paper contains the names of Orville and Wilbur Wright, along with the words "flying machine" to describe the world's first successful plane.

The Wright brothers made history with their first controlled, powered human flight back on Dec. 17, 1903. The patent was filed approximately nine months before the device's first flight and was granted in 1906.

"Be it known that we Orville Wright and Wilbur Wright, both citizens of the United States, residing in the city of Dayton and state of Ohio, have jointly invented a new and useful machine for navigating the air," it reads.

Although the successful flight of the airplane lasted just 12 seconds and covered 37 meters, it was considered a huge step forward for the human race.

"If somebody puts something back in the wrong place, it's essentially lost," said William J. Bosanko, the National Archives' boss. "In this case, we didn't know. We had to ask ourselves: 'Is it something that could have been stolen?'"

The rediscovered Wright brothers "flying machine" patent file will be put on display at the National Archives Museum later this month. Among the items still missing from the archives are Eli Whitney's cotton gin patent and letters written by Abraham Lincoln.