The truth is the green flash does exist and its cause is well understood. Just as the setting Sun disappears completely from view, a last glimmer appears startlingly green... The effect is typically visible only from locations with a low, distant horizon, and lasts just a few seconds... the effect is caused by layers of the Earth's atmosphere acting like a prism.
There is a gallery of photos in Wikipedia; the one embedded above comes from the archives of Astronomy Picture of the Day, a wonderful source for photos of both astronomical and atmospheric phenomena.
Link.