How the Twin Towers were brought down on 9/11

It was a controlled demolition, but not with thermite, since the buildings were complete steel beam buildings. As incredible as this sounds, it was an underground thermo-nuclear explosion, which caused the buildings to crumble into dust due to the extremely high temperatures. This was the reason for the pyroclastic clouds, and the upper part, that was left, going down in free fall. Moreover, you would have expected a huge heap of ruins after these buildings collapsed, but instead you had a blazing hot crater, where the nuclear device was positioned (about 50 meter under the buildings). The WTC complex had a unique (nuclear) demolition schema back in 1968 (a triple 150 Kt nuke), which allowed to get a building permit in New York, so it could be knocked down if necessary.

The picture shows the principle.