The Hessdalen valley in 1953, decades before residents began reporting the lights National Archives of Norway (Riksarkivet), CC BY-SA 4.0, via Wikimedia Commons
Residents of Norway's Hessdalen valley began reporting strange lights in December 1981, up to about 20 a week into 1984. Project Hessdalen ran an instrumented field study in 1984, and Ostfold University College set up an automatic measuring station in 1998. Their cause is still debated.
