It would seem, to any rational person, that an underground attack on a very tall building would require some plan to evacuate people trapped above the ground floor!
People ABOVE the ground floor survived almost entirely.
What is it you don't understand?
People survived below impact. Above impact which was WAY UP THERE, did not.
Why do you not understand this?
it did - BUT NOT IN THE EVENT OF PLANES FLYING INTO THE BUILDINGS AND CUTTING THE STAIRWELLS.