Answer: Algae Spores

Throughout both ancient and recent history there have been reports of “blood rain”, rain that falls with a red tint. Over the centuries, many theories were proposed as to the origin of the rain. Most ancient scholars attributed the appearance of the rain to the gods (though, in Cicero’s defense, he suggested, with some foresight, that the red rain came from an earthly contagion). Well into the Middle Ages, a common belief was that blood rain really was blood and that it was created when the blood washed from battles and other sources (like animal slaughter) contaminated water sources that supplied the clouds. The concept of distillation and evaporation was poorly understood, obviously, as the rain cycle would just leave the water bloodier and not carry the blood up into the sky.

By the 19th century, people were examining the phenomenon a little more scientifically and the going theory was that the coloration was caused by dust kicked up by storms. Well into the late 20th century, the red tint was attributed to the presence of iron oxide in the aforementioned dust.

It wasn’t until a study conducted in 2015 that we had conclusive proof of the origin of blood rain. Researchers collected samples of blood rain, analyzed them, and found that they contained aerial spores of the green microalgae Trentepohlia, and that they were responsible for the coloration.