A giant hole that appeared in a uniform layer of cloud over Mobile, Alabama, in the US, has left scientists puzzled. 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...