On 29 November 1989 two gendarmes on a road outside this town in German-speaking Belgium stopped in front of a large triangular object with three lights at its corners and a searchlight beam pointing down. It moved over the forest more slowly than an aircraft can fly, and without sound. Over the next year and a half more than two thousand reports with the same description were collected across the country. On the night of 31 March 1990 the Belgian Air Force scrambled two F-16s on ground radar data; their onboard radars locked on to a target that broke lock with accelerations beyond what the aircraft could follow. The Belgian Air Force made the material public — a rare piece of openness. No explanation was ever found.

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The Lubbock Lights

On the evening of 25 August 1951 three professors from Texas Tech were sitting in a garden in Lubbock when a semicircle of bluish lights passed silently overhead. Over the following weeks the formation came back dozens of times, and a student, Carl Hart Jr, photographed it — five frames that went round the country. Project Blue Book examined the negatives, found no sign of a fake, and offered a plain explanation: plovers flying in formation and lit from below by the town's new mercury-vapour streetlights. The professors objected that they had seen the formation before the lights were installed. The argument was never settled, and Hart's photographs remain among the most reproduced in the subject.