An unexplained bizarre phenomena occurred in the small village of Biskopsberga, Sweden on May 16, 1808. Chances are it is an extremely rare natural occurance that has never been repeated elsewhere, although a handful of similar instances have happened, none are quite the same.
On a warm spring afternoon, without a cloud in the sky the sun was blocked out by millions of minute spheres, basically bubbles barely a few inches round. People reported that they could stare right at the sun without the slightest smidgen of discomfort one would associate with staring into the center of our solar system. The mass of spheres appeared over the village from the western horizon.
The bubbles were dark brown and appeared to be headed towards the sun. The closer they got to the Sun the darker they appeared. Eventually they changed course somewhat, perhaps with the shifting wind and proceeded eastward all the while a steady stream of new spheres poured in from the west.
As they approached the sun, their speed diminished but they quickly picked up the pace once they passed the face of the sun. They eventualy disappeared over the eastern horizon.
As per E. Acharius of The Swedish Academy of Sciences 'the phenomenon lasted uninterruptedly, upwards of two hours, during which time millions of similar bodies continually rose in the west, one after the other irregularly, and continued their career in exactly the same manner.'
K.G. Wettermark, secretary of the Swedish Academy of Sciences. was one of the witnesses to the event. Wettermark made note that just before the spheres landed, they looked like 'air bubbles which children use to produce from soapsuds by means of a reed. When the spot, where such a ball had fallen, was immediately after examined, nothing was to be seen, but a scarcely perceptible film or pellicle, as thin and fine as a cobweb, which was still changing colors, but soon entirely dried up and vanished.' So basically as per Mr. Wettermark although they looked somewhat like soap bubbles their texture resembled cob webs. He also states they changed colors which sounds like what an oil slick does on the surface of water.
What ever these bubbles were may never be known as the incident has never repeated itself so far as we know. In 1566 a similar incident did occur in Basil Switzerland where observers reported the spheres were engaged in an aerial battle. Assuming it was the same type of phenomena it would also be safe to assume that the 'aerial battle' reported in the Swiss incident may have been due to more tumultuous winds than in Sweden.
'At the time when the sun rose, one saw many large black balls which moved at high speed in the air towards the sun, then made half-turns, banging one against the others as if they were fighting a battle' The Swiss incident, like the Swedish one lasted several hours.