U.S. Government Textbook (Chapter):

“CHAPTER XIII. UNIDENTIFIED FLYING OBJECTS, 
INTRODUCTORY SPACE SCIENCE – VOLUME II – 
DEPARTMENT OF PHYSICS – USAF (Washington, D.C.)”, 
(NICAP.org)

The U.S. Air Force concludes that the UFO phenomenon appears to have been global in nature for almost 50,000 years, according to the textbook:

http://www.nicap.org/af_doc/AFPhysicsChapter.pdf

Link to transcribed version: 

http://www.cufon.org/cufon/afu.htm

(wikimedia.org image)