The Fallstreak Cloud is a hole made in a cloud naturally. These holes are made because of the cloud’s water temp. being below freezing, but have not frozen yet. Then, when ice crystals do start to form, the water droplets along the ice crystals begin to evaporate, causing the “punched hole” look in the cloud. (This could also be an answer to some UFO sightings) NATURE IS SO COOL.
Photo source: cloudappreciationsociety.org