Sightings of this particular phenomenon go back as far as 1811 and occur in the Hessdalen Valley, which is in the middle of Norway. However, in the early 1980s, the lights became much more frequent, with a peak of nearly 20 separate reports each week. Since then, the sightings have decreased in frequency, with observations numbering 10–20 per year. Normally, the lights are either bright white or yellow and hover above the ground.Various scientific studies have been commissioned to find out the source behind the lights, but no conclusive explanation has been found. (There are cameras stationed around the valley, set up to take pictures of any bright light.) Studies meant to refute some of the findings have pointed out a variety of logical explanations, including car headlights and mirages, though they admit that such things don’t necessarily explain every occurrence.
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From @sharoncole