“A fallstreak hole, also known as a hole punch cloud, punch hole cloud, skypunch, canal cloud or cloud hole, is a large circular or elliptical gap that can appear in cirrocumulus or altocumulus clouds. Such holes are formed when the water temperature in the clouds is below freezing but the water has not frozen yet due to the lack of ice nucleation (see supercooled water). When ice crystals do form it will set off a domino effect, due to the Bergeron process, causing the water droplets around the crystals to evaporate: this leaves a large, often circular, hole in the cloud.”
Deutsch: Hole-Punch Cloud.
I first stumbled upon this hole punch cloud, which was photographed in Korumburra, Australia, a few days ago. It has a sort of “rainbow” inside, which is explained in this National Geographic article from last year: Explaining Rare ‘Hole Punch’ Cloud With Rainbow in the Middle.
A Google image search finds lots of other great photos.