Unusual natural phenomena are likely to account for a number of so- called UFO encounters. Among the most intriguing are "earthlights", first properly studied in the mid-1970s.
In the UK, Paul Devereux and Andrew York mapped out areas of UFO activity in Leicestershire, comparing them with maps of tectonic strain along geological fault lines. Two years later Michael Persinger and Gyslaine Lafrenière carried out a similar comparison of the US. Both teams found striking correlations between areas of active tectonic pressure and accounts of unusual lights in the sky.
One of the mechanisms suggested by Persinger and Lafrenière to account for the lights was a form of piezo-electrical effect. This occurs when crystals are crushed under pressure, producing electricity and sometimes ionizing the air above to create glowing lights. These might be considered supernatural or extraterrestrial.
These lights are assumed to be plasmas - gases in which some of the atoms have become ions and sometimes called the "fourth state" of matter. Such airborne plasmas glow brightly at night and appear silvery and metallic during the day - just like flying saucers.
One of the mysteries about these plasmas is that they have been described as behaving "intelligently". Intelligent gases have been the subject of only the most far-out science fiction, but research by Mircea Sanduloviciu at Romania's Cuza University has suggested that life may, indeed, be a gas.
Sanduloviciu wanted to recreate the conditions on Earth before life began, in which the electrified atmosphere was conducive to plasma formation. He found that, when charged, the negative electrons in the gas atmosphere - in this case argon - formed a spherical outer layer of up to 3cm across, the positive ions created an inner layer, and the remaining gas atoms would form a nucleus at the centre.
Such a distinctive outer boundary layer is one of the four attributes that define living cells. According to Sanduloviciu, the plasma spheres also display the other three: they replicate - by splitting in two - grow, and even "communicate" via electromagnetic energy. They could, he thinks, have been the first living matter on the planet.
Might the earthlight spotters be encountering their descendents?