UFO's are a very real phenomina regardless of what we actually think about them. Here is a link to some US military and FBI documents that date back from 1948 up to the 1980s. They are a tiny fraction of the many ducuments relating to UFOs over ICBM sites and air force bases but it gives you an idea of how seriously this was taken by the authorities at the time. The USA being the worlds primary nuclear power seems to have been the main point of interest of these UFO probes over many decades.