Since at least the 1930s, residents of and visitors to the valley of Hessdalen, Norway, have reported strange balls of light in the sky above.
While it's easy to dismiss reports as UFOlogy nonsense (especially when some write "papers" questioning whether the lights could actually be wormholes) and some sightings are actually of explainable phenomena such as "planes", the lights have been witnessed reliably throughout the years, in the day and night, and have been photographed extensively.
According to computer scientist Erling Strand – one of a small team from the University of Oslo, the University of Bergen, and the Norwegian Defence Research Establishment who attempt to document and explain the lights – the lights stopped suddenly in 1983, before returning with gusto in 1984. Strand and his team used radar, a magnetometer, a radio-spectrum analyzer, a seismograph, cameras, a Geiger counter, and an infrared camera to assess the phenomenon, before establishing a permanent observatory to capture further incidents.
The lights, according to the data collected as well as witness reports, can move slowly, or sometimes dart about and follow a random path, and can last from a few seconds to over an hour. Where size has been estimated, they have been described as being the size of a car.
What is causing them? Unfortunately, we don't know for certain, but there are a number of interesting (and fun) theories.
One theory, put forward in a 2010 paper published in the Journal of Atmospheric and Solar-Terrestrial Physics, suggests that the phenomenon is caused by radon decaying in the atmosphere, given that Hessdalen valley (and Norway as a whole) has one of the highest radon concentrations in Europe.
While cool, an even sexier idea first proposed in 2006 is that the phenomenon could be caused by the landscape acting as a natural battery, which then discharges at regular intervals. The Hessdalen valley is split in two by a river, with zinc and iron-rich rocks on one side of the river and copper-rich rocks on the other. The anode of this "perfect natural battery" would be the zinc/iron section, while the copper half would be the cathode.
The only thing missing is an electrolyte solution to transfer charge between the two sections of the battery.
"The missing piece to support the 'natural battery model' was identified in 2012, with the re-discovery of the local sulphur mines. Active until 1933, they were closed because the mining company bankrupted," a team wrote in 2013.
"We now hypothesize that the torrents flowing out of the mines and into the Hesja river might contain sulphuric acid"
The team noted that anomalies in the magnetic field have been detected in the area, consistent with a natural battery. It's not known how a natural battery would produce enough charge to produce such visible lights, however, the team "speculatively suggests that the local characteristics of the valley lead to the production of cold plasmas and/or ion bubbles".
While an interesting possibility, there is still no overall consensus on what is causing the lights, nor whether the valley is acting as a gigantic battery, perhaps as big as the legendary single A.