In some forests on humid winter nights, peculiar ice crystals form on rotting wood.
The ice looks like bursts of hairy cloud, and sometimes a bit like candy floss.
It is neither. These hair-like wisps appear at night and melt when the sun comes up.
Scientists have now discovered exactly what gives "hair ice" its strange shape. It's caused by a fungus called Exidiopsis effusa.
Hair ice only grows on humid winter nights when the temperature is just below 0°C.
You can see it grow in the time-lapse video below:
The ice is incredibly hard to spot. If there is any snow around, it camouflages it.
"When we saw hair ice for the first time on a forest walk, we were surprised by its beauty," says Christian Matzler of the University of Bern in Switzerland.
Their curiosity piqued, Matzler and his colleagues started investigating.
The team has now published their findings in the journal Biogeosciences. Their analysis supports a 100-year-old theory on where the ice came from.
When the fungus is not present, ice still forms, but in a crust-like structure instead.
"The action of the fungus is to enable the ice to form thin hairs – with a diameter of about 0.01mm – and to keep this shape over many hours at temperatures close to 0°C," says Matzler.
He suspects that the hairs are stabilised by a "recrystallisation inhibitor" provided by the fungus.