Kerala's red rain
From 25 July 2001 red rain fell on parts of Kerala. Indian botanists said algae; two physicists proposed cells carried by a comet.
Wikipedia reports that coloured rain began on 25 July 2001 in Kottayam and Idukki, with yellow, green and black rain also reported. Many more falls were reported over the next ten days, then fewer until late September. Locals said the first fall followed a loud thunderclap and a flash of light, and that groves of trees then shed shrivelled grey leaves. Wells were also reported to have vanished or appeared suddenly. The rain fell over small areas, a few square kilometres at most, sometimes with ordinary rain a few metres away, and each fall lasted under 20 minutes. Each millilitre held about 9 million red particles, which Wikipedia says implies about 50,000 kilograms in all.
Boing Boing reports that Godfrey Louis and Santhosh Kumar, physicists at Mahatma Gandhi University, posted a paper arguing that a meteor had left biological cells from outer space. It says they claimed the cells reproduced at temperatures as high as 300 degrees and carried no DNA, and that these claims have never been verified in a peer-reviewed publication. Skeptoid takes the opposite side. It says Indian botanists had no doubt the particles were algae and confirmed it within a week or two, and that most later articles promote the alien theory without mentioning the algae.
"The coloured rain of Kerala began falling on 25 July 2001, in the districts of Kottayam and Idukki in the southern part of the state. Yellow, green, and black rain was also reported.[3][4][5] Many more occurrences of the red rain were reported over the following ten days, and then with diminishing frequency until late September.[4] According to locals, the first coloured rain was preceded by a loud thunderclap and flash of light, and followed by groves of trees shedding shrivelled grey "burnt" leaves. Shriveled leaves and the disappearance and sudden formation of wells were also reported around the same time in the area.[12][13][14] It typically fell over small areas, no more than a few square kilometres in size, and was sometimes so localised that normal rain could be falling just a few meters away from the red rain. Red rainfalls typically lasted less than 20 minutes.[4] Each millilitre of rain water contained about 9 million red particles. Extrapolating these figures to the total amount of red rain estimated to have fallen, it was estimated that 50,000 kilograms (110,000 lb) of red particles had fallen on Kerala.[4]"