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A mysterious and deadly natural phenomena called superfog has been made in the lab for the first time.

Formed when smoke and moisture mix to make a murky dense haze that is impossible to see through, superfog is rare. But it鈥檚 been blamed for several lethal pile-ups on roads in the United States.

Five people died and 38 were injured when superfog caused 70 cars to smash together on the I-4 highway in Florida in January 2008. 鈥淚t was clear, it was a little foggy, then it was total darkness,鈥� a driver involved in the accident said at the time.

The interstate pile-up was blamed on deliberate controlled burning of nearby brushland 鈥� a landscape management tactic intended to reduce fire risk and improve habitat for wildlife. The US burns a combined area the size of Switzerland in this way each year.

Researchers have struggled to understand what turns regular smoke produced from such fires into occasional dangerous superfog. The new experiments to set fires in different lab conditions suggest it鈥檚 down to a combination of chilly nights, humidity and soggy fuel.

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Overnight smouldering of controlled fires produces large amounts of water vapour, which condenses on smoke particles into concentrated clouds of unusually small droplets, the research shows. These smaller and densely suspended water drops scatter more light and so reduce visibility so drastically.

This could be the first step to finding a way to reduce the risk during controlled burns. But Marko Princevac, a mechanical engineer at the University of California, Riverside, who led the research says it鈥檚 still early days.

鈥淣ow we know what are the proper mixtures of various ingredients that form superfog,鈥� he says. 鈥淏ut we still do not know how to predict when those ingredients in the right mixture will form.鈥�