Since at least 1981, luminous unidentified objects have appeared regularly in the Hessdalen valley, Norway. A 1984 field expedition by Project Hessdalen deployed radar, magnetometers, seismographs, and spectrum analyzers over five weeks and recorded 53 separate phenomena, some simultaneously detected on radar while invisible in optical wavelengths.

The project was initiated by Erling Strand with backing from UFO-Norge and UFO-Sverige. Radar returns indicated objects moving at velocities up to 8,500 m/s. In 1998 an automated monitoring station was established, and later research by Østfold University College and the Italian National Research Council measured radiant power reaching 19 kW. As of 2026 no consensus explanation exists.