Thousands of pages of UFO files related to US Air Force investigations between 1947-1969 have been released this week, and you can search for them over at archives of the Project Blue Book Collection. More than 700 cases (5.5% of the files) remain unsolved.

Here are 7 of the photos, some of which have been explained, and some which are still great mysteries:

Nine UFOs over Mount Rainier, on 24 June 1947 by Kenneth Arnold. The US government agencies took an interest in reports.