Most paranormal researchers assume that UFOs are spacecraft piloted by beings from other planets or even other dimensions. A few investigators, however, have postulated that these strange objects may actually be living creatures-- another type of Atmospheric Beast like the unknown entities that may produce star jelly (which I mentioned briefly in the Crawfordsville Monster entry).
One intriguing incident which may support this view happened in Septemnber, 1950. Two Philadelphia police officers, Joe Keenan and John Collins, spotted a glowing mass drifting towards the ground in an open field. Upon reaching the touch-down site, the officers reported finding a six-foot wide domed mound of what appeared to be purple jelly, so light that it barely even bent the stalks of the weeds it had landed on. The mound was faintly luminescent and appeared to pulse faintly like a living thing. According to some reports, part of the thing had even begun to crawl up a nearby telephone pole.
Keenan and Collins called for backup and once the other men had arrived, all four decided to try to move the thing. Collins, however, found that when touched, pieces of the glob broke off like foam and quickly evaporated, leaving behind an odorless, sticky residue on his hands. Over the course of the next half hour, the glowing purple glob slowing dissolved until nothing was left to mark its brief existence.
The story of the Philadelphia glob doesn't end there, though. Seven years later movie producer Jack H. Harris tasked his friend Irvine Millgate with coming up for an idea for a marketable film. This being the 1950s, science fiction monster movies were huge. So Millgate used the Philadelphia Glob incident as the basis for a story of a malignant extraterrestrial jelly-monster that terrorizes a town in Pennsylvania. Originally intended only as a cheesy B-movie to make a little cash, Millgate and Harris' film, 1958's The Blob, became a hit and helped start the career of its star, Steve McQueen.
Not to mention creating a pretty catchy earworm: