Some of you may have seen these already--I hadn't. The Pentagon shot is particularly fascinating, and to me, revealing.

I had heard reports that the plane colliding with the building had punched through all the outer rings to the center. Now, I am obviously no expert, but this does not look to be the case. While the outer ring has collapsed at one point, the inner rings look to be structurally intact, at least from the outside. This building's walls must be made of some tough stuff--I think reinforced concrete.

This shot also makes clear that the fires spread widely through the wooden understructure ("attic") of the peaked roof in those areas that are built that way (the gray parts). What looks to be flat, composite-type roof did not suffer as much damage. This may explain why they have had so much difficulty extinguishing the fires.