This book reports on and investigates more than a hundred mid-air encounters between civil and military aircraft and something else that was flying unidentified within our atmosphere. The cases span fifty years and come from all over the world. Since 1991 at least eight such encounters have occurred in Britain alone. They involve radar trackings, visual sightings by skilled airline pilots and including baffling mysteries of both near-misses and total disasters.Jenny Randles tackles a very real and decidedly terrestial mystery that is a potential threat to all who travel by air today. The author assesses the reliable evidence acknowledged by the proper authorities, addresses the various riddles, and shows beyond any doubt that there is a puzzling phenomenon that endangers the safety of every aircraft in our crowded aviation system.