Absolutely natural, and really cool to see! I've seen lines of them, in their last stage before dissipating. They are also called "Fallstreak
holes". They form from a chain reaction of super-cooled water turning into ice crystals (the Bergeron process), which fall, leaving streaks called
virga. The formation beneath is sometimes called a sylph, which some people believe is a being of the air, come to rid our skies of "chemtrails".
It is just a cloud, though. And because the hole forming above it is developing symetrically, so will the cloud below. It is believed that the
moisture from planes is one of the catalysts to start formation; so planes really do effect the clouds, if only for a little while.
The "UFO" cloud that someone is bound to post about is an optical illusion.
Almost forgot, here's a really nice one. This is the Cloud Appreciation Society, which also has 102 pictures of them, much of them more bizarre than
this one. My favorite looks like a jellyfish. The link is a picture of one; to see the others go to "Picture Gallery", "Select Category",
"Fallstreak Holes". These people are really sure these are all clouds, too.
edit on 9-1-2011 by stars15k because: Forgot something