A giant hole that appeared in a uniform layer of cloud over Mobile, Alabama, in the US, has produced some intriguing photos.
By Dr David Whitehouse
BBC News Online science editor
Local resident Joel Knain said as he took pictures: "I immediately realised that I was seeing something unique."
Meteorological experts believe the hole formed when ice-crystals from a passing plane fell through the cloud, causing the water droplets in it to evaporate.
Experts say the process involved is related to that of cloud seeding, which is used to make rain over crop fields.
Stuck inside of Mobile
The unusual phenomenon was observed on 11 December last year.
"I ran inside to get my camera and shot-off 10-12 frames to capture the scene," Joel told BBC News Online.
"I would guess that we stood there for 10-15 minutes just staring in amazement."
Strictly speaking there is no scientific term for the apparition, and what exactly it is has been the subject of much meteorological speculation.
One hypothesis is that the hole is made by falling ice-crystals that could have come from the exhaust of a passing aircraft.
It is possible the air was at just the right temperature and with just the right moisture content so that the falling crystals could absorb water from the air and grow.
The moisture removed from the air could have increased the evaporation of the cloud's water droplets, which then disappeared to produce the dramatic hole.
The wispy clouds seen below the hole may be heavier ice-crystals that have fallen from the hole, evaporating (the correct term is subliming) before they reach the ground.
WKRG-TV in Mobile carried pictures of the hole, and some of the images have been posted on the station's website.
Image copyright Joel Knain