In a film clip that is being shared on internet all over the world, you can see what Jonas Landmark and Karin Bäckborn saw that day.
"It has been an amazing skating season in Jämtland this year, and we have long spoken about going to Blanktjärn. Often there is a bit of snow or frost on the ice, and then it won't be see-through. But now the conditions were perfect," Landmark told Swedish Television News.
The lake is unusual because of the turquoise colour, which has been created due to a chemical reaction. But the fact that you can see straight through the ice is also very rare.
Jonas Landmark told the local news paper Östersundsposten: "You could see all the way to the bottom of the lake. It is pretty deep, some 10-15 metres, and the big fish that you can see in the film clip is probably ten metres down. It was like being in an aquarium," he said.