When I first saw this photo I was amazed and confused, I had never seen anything like it , is it real, is it caused by a volcano or tornado or what? So of course I Googled it. Looking at it I can understand why people might think it was a portal of some kind or the end of the world.
photo by~Joshua Earth & Space Museum
"How does the hole occur in the clouds ? - the puffy clouds were likely composed of ice crystals and very small super-cooled liquid water droplets. Studies have shown that very small supercooled liquid water droplets can exist well below freezing (as low as -40 degrees (C, or Centrigrade) ). The temperature in the cloud layer was around -24 deg (C) at the time of occurrence. Sometimes...when ice and the supercooled liquid droplets coexist within a cloud...a process occurs whereby ice crystals grow at the expense of the droplets. If the mixed cloud is agitated...the supercooled droplets freeze instantaneously. It is understood through microphysical arguments that water vapor must evaporate from the surrounding supercooled droplets and flow towards the ice crystals. This would start a cycle whereby newly formed ice crystals cause the surrounding droplet to evaporate...resulting in a circular form void of clouds with a mass of crystals in the center. Many photos taken that day seem to represent this process."
explanation by http://www.weathervortex.com/sky-holes.htm