On December 24, 2012, a strange double helix cloud was spotted just outside Moscow, Russia.
A series of amateur photographs were submitted to a Russian site called netall.ru (scroll down to Dec. 24) on the morning of the occurrence.
While it is not entirely known what caused such a phenomenon, there is a fairly extensive investigation under way on the website metabunk.org which believes it to be some kind of contrail.
Contrails or vapour trails are long thin artificial clouds that sometimes form behind aircraft.
Their formation is most often triggered by the water vapour in the exhaust of aircraft engines, but can also be triggered by the changes in air pressure in wingtip vortices or in the air over the entire wing surface. Like all clouds, contrails are made of water, in the form of a suspension of billions of liquid droplets or ice crystals. Depending on the temperature and humidity at the altitude the contrail forms, they may be visible for only a few seconds or minutes, or may persist for hours and spread to be several miles wide.