At first, the FAA attributed the incident to some kind of weather phenomenon, and United Airlines advised its employees not to talk about it, according to the Tribune.
"If this had been a plane, it would have been investigated," Hilkevitch says. "The FAA treats the smallest safety issue as very important."
To explain the witnesses' reports, he offered his best "guess." They may have seen a "hole-punch cloud," he said, which is in "a perfect circular shape like a round disc" and has "vapor going up into it."
These unusual natural cloud holes form only at below freezing temperatures, according to climatologists. It was 48 degrees at O'Hare that afternoon.
John Callahan, Division Chief of Accidents and Investigations for the FAA during the 1980s, says it's not at all surprising that the O'Hare UFO was undetected on radar.
Radar technology cannot always capture objects at extremely high speeds. A hovering object wouldn't necessarily show up either. "If it did, it would be a small dot, and air traffic controllers would not give it much concern," Callahan says.
One of the things I learned in grad school: Sometimes it is better to say "I don't know" when you don't actually know.
Too bad it isn't as catchy as Swamp Gas.