An unidentified flying object (UFO) is any perceived aerial phenomenon that cannot be immediately identified or explained. On investigation, most UFOs are identified as known objects or atmospheric phenomena, while a small number remain unexplained. Scientists and skeptic organizations such as the Committee for Skeptical Inquiry have provided prosaic explanations for a large number of claimed UFOs being caused by natural phenomena, human technology, delusions, or hoaxes. Small but vocal groups of ufologists favour unconventional, pseudoscientific hypotheses, often claiming that UFOs are evidence of extraterrestrial intelligence. Beliefs surrounding UFOs have inspired parts of new religions. While unusual sightings have been reported in the sky throughout history, UFOs became culturally prominent after World War II, escalating during the Space Age. The 20th century saw studies and investigations into UFO reports conducted by governments (such as Project Blue Book in the United States and Project Condign in the United Kingdom), as well as by organisations and individuals. History Early history before the 20th century People have observed the sky throughout history, and have sometimes seen unusual sights, such as comets, bright meteors, one or more of the five planets that can be readily seen with the naked eye, planetary conjunctions, and atmospheric optical phenomena such as parhelia and lenticular clouds. One particularly famous example is Halley's Comet: this was recorded first by Chinese astronomers in 240 BC and possibly as early as 467 BC. As it reaches the inner solar system every 76 years, it was often identified as a unique isolated event in ancient historical documents whose authors were unaware that it was a repeating phenomenon. Such accounts in history often were treated as supernatural portents, angels, or other religious omens. While UFO enthusiasts have sometimes commented on the narrative similarities between certain religious symbols in medieval paintings and UFO reports, the canonical and symbolic character of such images is documented by art historians placing more conventional religious interpretations on such images. Some examples of pre-modern observations of unusual aerial phenomena: Julius Obsequens was a Roman writer who is believed to have lived in the middle of the fourth century AD. The only work associated with his name is the Liber de prodigiis (Book of Prodigies), completely extracted from an epitome, or abridgment, written by Livy; De prodigiis was constructed as an account of the wonders and portents that occurred in Rome between 249 and 12 BCE. An aspect of Obsequens'