'Blood rain' falling from the sky in Spanish town challenges scientists' imagination

In "My Fair Lady," when Professor Higgins and Colonel Pickering exhaust themselves trying to get Eliza to correctly say "the rain in Spain stays mainly in the plain," they probably weren’t talking about this type of rain.

Residents in the Spanish city of Zamora reported last year that what appeared to be raindrops of blood were falling from the sky — leading to fears that some hazardous chemical was pouring down on them, or worse, that an Old Testament plague was about to strike.

After receiving a few samples of the so-called blood rain, scientists at the University of Salamanca were able to run some tests on the substance and determined that what fell on Zamora was something far less nefarious. The liquid turned out to be rain water infused with Haematococcus pluvialis, a freshwater green microalgae that’s capable of synthesizing a red carotene pigment called astaxanthin.

While the mystery of what the red liquid was had been solved, there was still confusion over how it got there.

The microalgae is not found in Mediterranean climates, so scientists' top hypothesis is that it may have been blown across the ocean by westerly winds, possibly from North America.

Events like what occurred last year in Zamora are rare to be sure, but not unprecedented. Since the 1890s, there have been similar bouts of "bloody rain" in the south Indian state of Kerala and the neighboring island nation of Sri Lanka, with the most recent occurrence happening in 2013.