2006, November 7, Chicago, Illinois; O’Hara Airport, pilots. crew and passengers of United Airlines witnessed a large metallic craft at 4:15 p.m. near gate C-17.
Although reported by pilots and crew, photographed by one person at least (based on recording between FAA and United) and at about 1000 ft. over C17 gate when reported to the FAA the response was jokes and ribbing about getting the holiday party started early.
[See the alleged photo at Chicago O’Hare Airport UFO Sighting 2006 | UfoSightingsToday]
Despite the laughter, the report was repeated, as more witnesses stepped forward. The object reportedly disappeared by going straight up.
Very quickly the authorities explained away the sighting as a weather phenomenon; a rare hole punched cloud or fallstreak cloud.
This explanation would have produced numerous witnesses. This is due to the observed fact, found in several articles on the phenomena, that hole punch clouds (and similar sub-varieties) generally persist for as long as four hours.
Investigators gained access to the actual recording of the phone calls between United and the FAA office and it reveals some interesting things. It emphasizes the ready ridicule reporting anything out of the ordinary garners.
The taped conversation between FAA and United seems, however, also appear to contradict that hole punch theory on several points. In that conversation, the report of the object by the United desk had others (FAA and others) looking for it above Gate 17 but they reported they could not see anything.
An article in Popular Mechanics (How Airplanes Punch Holes in Clouds - How Airplanes Increase Snowfall Around Airports (popularmechanics.com)) from 2011 indicates they dissipate after about 4 hours. If that is correct, then right after the report others should have still be able to see the hole punch cloud and verify what it was. Instead, witnesses reported the object seen left by rising straight up and disappearing from view.
See a photo of hole punch cloud at NOAA - NOAA Photo Library > Collections > National Weather Service > Other or on Flicker (Hole punch cloud | _ Nemo _ | Flickr)
Oddly, although these rare clouds come in many shapes and sizes, few of the photos can really be mistaken for anything but a cloud or weather formation object.
At the time witnesses were upset and worried that there was not more interest in the matter. It was only a few years post 9-11 after all and the idea of strange craft might have hovered over a major aviation center undetected was worrisome.
All early interviews with witnesses by the Chicago papers indicated the object had been seen for only a short time before it disappeared upwards. The local astronomer who suggested conditions were right for a hole punch to appear near the airport only suggested it but authorities ran with the idea desporte evidence to the contrary.
Chicago newspaper, January 2007 - In the sky! A bird? A plane? A ... UFO? -- chicagotribune.com (archive.org)