A 15 kilometre-long ice 'wall' has formed naturally on the open water Tehinselkä straight in the centre of Finland’s second largest lake, Päijänne.
'Open water' (Finnish: selkä) is an expanse of a large lake which is distant from the shore and also clear of any nearby islands or other obstructions.
The conditions for the creation of an ice wall on the lake occur about once a decade, when the ice is thick enough and there are large variations in the daily temperatures.
As the temperature on the lake can fluctuate by as much as twenty degrees within 24 hours, the huge ice sheet expands and contracts with the changing temperature.
As the ice melts, it creates a chasm, but when it freezes again and expands, it forces the edge of the chasm upwards, creating the 'wall' effect.
"I have seen these chasms form over the decades, but this is one of the largest," professional fisherman Jorma Kääpä told Yle.
The wall currently extends far enough to cover the territories of two provinces and three municipalities.