A truly vital piece of evidence that provides definitive proof that the World Trade Center towers were brought down by controlled demolition are the videos of yellow-hot molten metal seen cascading off the South Tower (World Trade Center Tower 2).
That piece of evidence isn't merely a smoking gun: it's a smoking nuclear cannon. Those videos, alone and by themselves, are irrefragable *proof* that the South Tower (at the very least) had thermite-like ("like" in the sense of producing comparable temperatures) incendiary demolition charges with the ability to easily slice through structural steel going off within it. There is no innocent explanation for what those videos record.
That is to say, the only way to get around that it is thermite which is causing that yellow-hot metal to cascade off the South Tower before its collapse would be to posit that we are seeing a different form of extremely powerful incendiary with thermite-like temperatures at work in the videos. Of which, even if true, would be every bit as much damning, since no such powerful indendiaries can be accounted for without involving a sinister intent to plant them there.
Below are videos which contain some of this footage:
"Shot from street level of South Tower collapsing," CameraPlanet http://video.google.com/videoplay?docid=-2991254740145858863http://amsterdam1.plunder.com/x/84738/supportthetruth.com-vids-thermite.wmvhttp://www.checktheevidence.com/911/Molten%20Metal%20from%20WTC.mpg
"Wtc 1, impact site close up, tower collapse close up, long shot, people shouting," CameraPlanet http://video.google.com/videoplay?docid=-8564772103237441151
From the color of the yellow-hot molten metal seen cascading off the South Tower, it had to be at least over 1000 °C, yet jet fuel burns in open air at 260-315 °C; nor do burning office, building, or plane materials impart temperatures anywhere near that hot to structural members (indeed, it would present quite a hazard if such articles were constructed with such powerful incendiaries, and so designers of such objects go out of their way to make sure that they are not). Thus, if it wasn't molten iron from thermite that we are seeing come off the South Tower, then by necessity a reaction source with a heat intensity very much like thermite had to be present. Yet there is nothing in the U.S. government's account that can explain such a heat source; indeed, there's nothing innocent that could explain it, since it requires some sort of extremely powerful incendiary.
For more on this, see Steven E. Jones's (Ph.D.; physicist and archaeometrist; former professor in the Department of Physics and Astronomy, Brigham Young University) below paper:
"Why Indeed Did the WTC Buildings Completely Collapse?," Dr. Steven E. Jones, Journal of 911 Studies, Vol. 3 (September 2006) http://www.journalof911studies.com/volume/200609/Why_Indeed_Did_the_WTC_Buildings_Completely_Collapse_Jones_Thermite_World_Trade_Center.pdfhttp://www.physics.byu.edu/research/energy/Papers/J6p2%20.doc
"Experiments to test NIST 'orange glow' hypothesis," Steven E. Jones, Ph.D., August 31, 2006 http://www.scholarsfor911truth.org/Experiments-to-test-NIST-orange-glowhypothesis.htmlhttp://www.scholarsfor911truth.org/Liquid_Aluminum_011.mpghttp://www.scholarsfor911truth.org/Liquid_Aluminum_012.mpg
"Experiments with Molten Aluminum," Steven E. Jones with Wesley Lifferth, Jared Dodson, Jacob Stevenson and Shannon Walch, circa June 2006 http://www.scholarsfor911truth.org/ExptAlMelt.doc
"A description of molten aluminum poured onto rusty steel," Wes Lifferth, Physics Shop, Brigham Young University, Journal of 9/11 Studies, Vol. 9 (March 2007) http://www.journalof911studies.com/volume/200703/Molten_Aluminum_Poured_onto_Rusty_Steel_by_Wes_Lifferth.pdf
Moreover, even the official FEMA scientists Jonathan Barnett, Ronald R. Biederman, and R. D. Sisson, Jr. bolster the evidence that thermate (i.e., thermite with sulfur added, which causes it to slice through steel even faster by forming a eutectic alloy with it) was used to bring down the WTC towers (see "Appendix C: Limited Metallurgical Examination" in World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations, Federal Emergency Management Agency, FEMA 403, May 2002 http://www.fema.gov/pdf/library/fema403_apc.pdf
Evidence of a severe high temperature corrosion attack on the steel, including oxidation and sulfidation with subsequent inter granular melting, was readily visible in the near-surface microstructure. ... No clear explanation for the source of the sulfur has been identified. The rate of corrosion is also unknown.
And in the below paper it is conclusively proved via chemical analysis using wavelength dispersive X-ray spectroscopy (WDS), scanning electron microscopy (SEM), and X-ray energy dispersive spectroscopy (X-EDS) that large quantities of thermite analogs (such as thermate) were used in the destruction of the World Trade Center towers:
"Revisiting 9/11/2001--Applying the Scientific Method," Dr. Steven E. Jones, Journal of 911 Studies, Vol. 11 (May 2007) http://www.journalof911studies.com/volume/200704/JonesWTC911SciMethod.pdf