There’s nothing quite like hair ice when you stumble upon it in the forest: strange, silky hair-like ice that grows from the wood of rotting branches, and until now scientists didn’t have a good explanation for how they happen or what causes them.

Physicist Christian Matzler at the Institute of Applied Physics at the University of Bern in Switzerland studied the rare phenomenon, having become fascinated by it ever since stumbling upon hair ice one time while walking through the forest, according to an R&D Magazine report.

Matzler and his colleagues have just published a new study in the journal Biogeosciences, confirming that a 100-year-old hypothesis was actually right: it’s because of a certain type of fungi growing in the wood.

His team studied wood samples collected over the course of three winter seasons from trees in Germany. They identified 11 species of fungi, and found that only one of them was present in every single sample: Exidiopsis effusa. For half of the samples, it was the only fungi that could be found.

But how does this fungi actually cause hair ice? Matzler and his team found that the driving mechanism for producing this sort of structure is ice segregation. In other words, liquid water near the surface of the tree branch suddenly freezes once it contacts the cold air, sandwiching a thin water film between the wood pores and the ice. Then, there is suction from the repelling intermolecular forces, and water in the wood pores freezes on the ice front, adding more layers. It’s the fungus, though, that gives it its structure.

Hair ice was first observed all the way back in 2018 by a German geologist, who assumed that it had something to do with fungus, although he was not able to identify which.