Fionn sez, "The fact is that the available film footage and stills of the damaged building are not wholly consistent with what many catastrophe analysts would have expected in such an incident – not least that much of the Pentagon damage was confined to two floors of the building."

For a description of the actual damage, and of the structure of and renovations to the Pentagon (which caused the damage to differ from what one would expect of a conventional building), see this page on the Snopes site: http://www.snopes2.com/rumors/pentagon.htm