It was just another gray day in Zamora, Spain. The rain fell as rain does, clear and cool. Nobody took much notice. 

And then it turned red. Within just a few hours, the rainwater collected in basins and fountains had gone from colorless to a murky, rusty red. Some residents worried that chemicals had been dumped into their water. Others suspected aerial saboteurs filling the tanks with contaminants. The religiously minded worried that the blood-like rain was a plague delivered by a wrathful god. 

Image Credit: Joaquín Pérez

They wouldn’t be the first to come to that conclusion. Blood rains, like rains of fish, are uncommon but not unheard of. In September 2001, red rain fell from the skies in Kerala, India, staining people’s clothes and prompting researchers to suspect both meteor dust and alien involvement. A government report settled on a cause much closer to home: airborne algae spores.

Elsewhere, the “blood” flow is perpetual. Red liquid continually seeps from a crack in Antarctica’s Taylor Glacier. But the blood of Blood Falls is neither blood nor pure, red water. Instead, it’s an iron-rich brine that oxidizes, or rusts, as it meets fresh air. 

As it turns out, the Spanish blood rain (which occurred in fall 2014) was similar to the rains in Kerala. A curious resident sent samples of the blood puddles to scientists at the University of Salamanca. The researchers examined the water and found particles of Haematococcus pluvialis, a green freshwater alga that turns red in times of chemical stress. The red color comes from a carotene pigment called astaxanthin. The scientists weren't totally shocked to find H. pluvialis in the water—its name literally translates to "blood rain algae."

Astaxanthin in H. pluvialis. Image Credit: Frank Fox via Wikimedia Commons // CC-BY-SA 3.0

But identifying the algae raised more questions. H. pluvialis is not native to the Zamora region and it can’t be found in any nearby bodies of water. The researchers concluded that the algae must have traveled far before its fall into the village fountains. They analyzed wind patterns and other meteorological data to trace it back to its source, but they still couldn’t pinpoint a location.

The team published their findings (and further questions) in the Spanish Royal Society of Natural History Journal.