Perhaps the most famous conspiracy theories surrounding Sept. 11 have to do with the most devastating attack that day: the planes that flew into the World Trade Center. Conspiracy theorists believe that the buildings would not and did not collapse as a result of being hit by planes, but rather by controlled demolition.
The documentary "Loose Change" argues the government planted explosives in the building ahead of time that ensured the collapse, saying that the steel structures would have normally withstood the impact and fire. However, others, including a Nova PBS documentary "Why the Towers Fell," argue that the unique construction of the towers and the way they were hit resulted in the collapse.
Theorists often point to the fact that jet fuel doesn't burn hot enough to melt steel. But others respond that the fire undoubtedly ignited other combustibles in the office building, like paper, carpet and electronics, that would have fed the fire. Also, the force of the impact, according to experts, probably blew off the fireproofing foam covering the floor trusses, and the force of the fire would only have to weaken the steel to cause a "pancaking" collapse.
Considering everything, in order to pull this elaborate conspiracy off, the government would have needed to use an enormous number of accomplices who are now keeping quiet. For many, this is harder to believe than the idea that 19 hijackers led by a known enemy of the U.S. were responsible.