"The “devil face” that many saw in the billowing smoke within seconds after Flight 175 smashed into the South Tower was almost certainly not Photoshopped [S1]. It is there, fully visible in the AP photo above, as well as on the CNN feed archived on ­YouTube, hovering below the point of impact, around the 70th floor. Of course, it helps to believe in Satan, as rendered by church artists during the Middle Ages, to fully appreciate the image. Also helpful in amplifying the effect is to listen to George W. Bush’s address that night, in which he said, “Today our nation saw evil.” There’s no doubt, there he is: Lucifer, a dead ringer for Osama bin Laden, sure as you breathe."

nymag.com/news/9-11/10th-anniversary/satans-face/

"One year later computer scientists Vladik Kreinovich and Dima Iourinski of the University of Texas at El Paso published a geometric analysis of the face in the photograph, also seen in a different image from CNN. The analysis showed that perturbations in the smoke can consist of horizontal lines (such as the “eyes” and “mouth”) and vertical lines (such as the “nose”) overlaid on a conic surface (the “head”). The scientists concluded that both the background shape (the cone) and the features on the background (horizontal and vertical lines) are naturally explained by the physics and geometry of smoke plumes emanating from fire."

blogs.scientificamerican.com/illusion-chasers/illusion-of-the-week-satan-in-the-smoke/

cs.utep.edu/vladik/2002/tr02-03.pdf