Since at least the 1930s, people in the Hessdalen valley in central Norway have reported unusual lights along a 12 km stretch. They are usually white, yellow or red, appear by day and by night, and can last from a few seconds to over an hour. Sometimes they hover, sometimes they sway slowly, and sometimes they move at great speed.
Between December 1981 and mid-1984, the lights were seen 15 to 20 times a week, and many people came to watch. Since 1983 'Project Hessdalen' has studied them, and in 1998 an automatic measurement station was set up in the valley. Østfold University College and Italy's National Research Council have run research there.
Ideas include burning dust from the valley's mines and rocks, and glowing clouds of charged dust in a plasma created by radon decay. By 2010, sightings had dropped to about 10 to 20 a year.
