It was complied based on people's evidences about strange flying objects over Ukraine. The 'secret files' comprise about 500 eyewitnesses' testimonies who saw UFO in Ukraine starting from the 17th century. According to UFODOS chief Yaroslav Sochka, the materials were collected from various sources, basically, Hydrometeorological Center of Ukraine Air Force and public ufological organizations. In August 2009, the UK National Archives released new UFO files containing a wide range of UFO-related documents covering the years 1981-1996 and describing more than 800 UFO cases.