Sky Jellyfish 

https://reymisterios.com/videos/ovnis/Un-agua-viva-gigante-flotando-en-el-aire-l2436.html 

(via Chrome translate)

The case occurred on a cold Sunday morning on November 17, 1974, when Mr. Laursen went for a walk with his dog near Lake Nørre Sø, in Viborg, Denmark. He liked to watch birds by the lake, so he had brought his 35mm camera out. He suddenly saw a huge object in the air, took the camera from it and immediately managed to take a photo. He then looked around looking for other people who could corroborate the incredible object he had photographed, but he couldn't find anyone.

When you turned your face back, the flying object was already gone. The image captured on the Kodacolor film, however, would already be one of the most curious physical tests. In 1979 the case was published in the magazine UFO-Kontakt, where an analysis Retired Hungarian Major Colman VonKevieczky claimed that the object was a "typical" discoid alien ship shrouded in fog. Various other ufological publications around the world would continue to claim over the years that the flying disc is like those of today using clouds as camouflage.