A witness who posted a YouTube video of a UFO from inside an airliner departing Amsterdam, Netherlands on March 17, has added a second, enhanced version of the video.

Skeptics are proposing that the object is a mylar balloon.  

If so, it would likely have been a large balloon, as the perspective scale of the object in the video does not add up to a typical, small balloon.

Keep in mind that the 747 aircraft (including the camera), was climbing to cruising altitude at a speed of at approximately 300-400 miles per hour.  

A jumbo jet would dwarf the size of balloon, and so to see a balloon that large from the window, would have to make it very close to the plane, and therefore it would fly by much faster.  

Conversely, a more plausible speculation is that the balloon was large, and good distance away from the aircraft in order to pass by so slowly at flight speed.

To see the original video, click here.