Project Skyhook was a secret US Navy program of the late 1940s to develop giant balloons for carrying out photoreconnaissance over the Soviet Union. Launched partially inflated, Skyhooks expanded as they rose until they reached their maximum size of up to 600 feet tall and 100 feet in diameter at an altitude of 60,000 feet. Almost certainly, they were behind many UFO sightings in post-War years, including the UFO chased, with fatal consequences, by Air Guard Captain Thomas Mantell.