What happened
On November 7, 2006, at around 4:15 p.m., a United Airlines ramp employee guiding a plane at gate C17 noticed a dark, round object hovering over the terminal just below the cloud base. Word spread quickly, and the object was watched by a dozen or so airline employees, including pilots and mechanics. After a few minutes the disc reportedly shot upward abruptly, leaving a distinct, circular opening in the clouds through which blue sky could be seen. The Federal Aviation Administration initially denied that anything had been reported, but after a Freedom of Information Act request it released a recording of a conversation with the tower. The story was broken by the “Chicago Tribune” on January 1, 2007.
Witnesses
A dozen or so United Airlines employees – ramp workers, mechanics, and pilots – and several witnesses outside the airport.
Explanations and hypotheses
The FAA attributed the event to a weather phenomenon, and the object did not appear on radar. Meteorologists suggested a so-called hole-punch cloud, which forms when a passing aircraft triggers precipitation in a supercooled cloud – although the temperature that day may not have favored this phenomenon. Other hypotheses include a balloon or reflected lights. No explanation has been conclusively confirmed.
Unexplained There is no convincing explanation.
More: Wikipedia (en)