Newspaper clippings, books and first-hand accounts of people who said they visited other planets are catalogued in a giant Swedish archive on paranormal phenomena, attracting the curious and researchers from around the world.

The Archives for the Unexplained (AFU) claims to be the world's biggest library of paranormal phenomena, with 4.2 kilometers (2.6 miles) of shelves running underground.

Clas Svahn, 65, and Anders Liljegren, 73, who run the archive located in the southeastern town of Norrkoping, say they are neither superstitious nor believers, but rather "curious investigators of the unknown".

The AFU—the name of both the library and the association that has collected documentation for more than 50 years—is mainly comprised of books, but also more original documents, such as first-hand accounts of paranormal activity recorded on tape and photos of ghosts.

"What we are building here at AFU is depository knowledge," explains Svahn, showing AFP journalists around the 700-square-metre (7,535-square-foot) library.

"We're trying to get as much as we can on... every kind of unsolved scientific mystery that we can find... to make this available for the world."

The library receives around 300 visits each year, by appointment only.