Keen gardener Kate Walker with one of the crabs which rained down on her home. She got the shock of her life when a freak storm rained 20 crabs down on her. Kate Walker was picking beans in her garden when she felt what she thought was heavy rain hitting her. Story continues.

We've heard plenty of tales about freak weather that are strange, but nonetheless true. In August 2000, a shower of sprats, dead but conveniently still fresh, fell from the skies onto the English port of Great Yarmouth just after a thunderstorm. A torrent of live toads pelted a Mexican town in June 1997. And in 2001, 50 tonnes of alien life forms rained down from the clouds over India.

Actually, I'm not sure that the alien story is true. But it is surprisingly persistent. I first saw it in 2003 in a scientific paper written by Godfrey Louis, a physicist working in the Indian state of Kerala, on the country's southern tip. He described how, during two months in 2001, red rain fell sporadically right across the state. No one could explain it, but after lengthy studies of red particles in the rainwater, Louis came to the extraordinary conclusion that they were alien microbes that hitched a ride to Earth on a comet.

Godfrey Louis - physicist

To most people, that would sound eccentric at the very least. It looked as if the idea would quietly wither on the vine. Then in January this year, it turned out that Louis's theory is still alive and kicking, and soon to roll off the press in a reputable peer-reviewed journal. I sent a preprint to several researchers, who despite voicing mixed opinions almost all agreed about one thing: the red particles Louis describes look biological.

"If they're not living cells, I don't know what they are," said Milton Wainwright, a microbiologist at the University of Sheffield, UK. "Maybe this is the beginning of something amazing." Another scientist simply commented: "Sounds like bullshit to me." That was it I could resist this weird and controversial story no longer.

The saga dates back to 25 July 2001, when red rain fell in a district of Kerala called Kottayam. Over the following two months, red rain fell sporadically there and in other Kerala districts, gradually tailing off over time. The local newspapers buzzed with eyewitness reports. People found their clothes stained by red raindrops. Although these usually had a mild red tint, sometimes the colour was so strong that witnesses compared it to blood. Usually, the red rain would fall for less than 20 minutes.

Louis, a solid-state physicist at Mahatma Gandhi University in Kottayam, was intrigued and decided to study the rain with his student Santhosh Kumar. The pair compiled more than 120 reports of the rain from local newspapers and other sources, and gathered samples of the red rain from spots more than 100 kilometres apart .

Under the microscope, they could see red particles 4 to 10 micrometres wide with an average density of about 9 million particles per millilitre. When they dried the samples they found that each cubic metre of rainwater contained about 100 grams of the red stuff. Louis suggests 5 millimetres of red rain would typically have fallen over a square-kilometre area during each of about 100 downpours. That would make 500,000 cubic metres of water in total, containing a staggering 50 tonnes of red particles.

What could they be? One possibility was that fine red sand had blown over Kerala from some distant desert. Sand can travel amazingly far. In July 1968, for instance, fine grit in raindrops left parts of southern England coloured rusty red. The sand had blown from the Sahara inside a massive high-pressure system before falling in a rain shower.

But under the microscope, the red particles that rained on Kerala were clearly not sand. Electron micrographs show that they are shaped like biological cells. "They don't look anything like sand, they look biological," says Monica Grady, a meteorite expert at the UK's Open University in Milton Keynes. The cells, if that's what they are, are mostly cup-shaped and have a thick wall.

One type of analysis shows their chemical make-up is about 50 per cent carbon and 45 per cent oxygen by weight, along with traces of other elements such as sodium and iron. That's consistent with the components of a biological cell, according to Jeffrey Walker, a molecular biologist from the University of Colorado in Boulder. But although many of the cells have some kind of detached inner capsule, there is no visible cell nucleus, and tests for DNA that Louis carried out came back negative.

Louis rules out a distant terrestrial source for the mysterious particles, because the red rain was concentrated over Kerala for two months despite changes in climate and wind patterns. Could the cells instead be local pollen or fungal spores washed off trees and houses by the rain? Louis says no, because red rain was collected in buckets placed in wide-open spaces. Equally, he says, the red particles can't be pollen or spores from the ground that accumulated in the atmosphere, because the rain would then have been red at the start of a shower; often the colour came later.

Instead, he links the coloured rain to a meteor airburst. During the early hours of 25 July 2001, just hours before the first red rain fell, several people in the Kottayam district heard a loud sonic boom that made their houses rattle. Louis has interviewed some of those who heard it, and concluded that it was too loud to have been an ordinary thunderclap. It's possible that an incoming meteor exploded in the atmosphere.

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