One of the most prominent conspiracy theories about the 9/11 attacks is that many conspiracy theorists believe that the World Trade Center towers came down as a result of explosives that were place in the building months in advanced. They base this belief on the comments and advocacy of a few engineers and demolition experts who believe that the buildings collapsed in a controlled demolition.
The major problem with this is that conspiracy theorists assume that a jumbo jet, nearly full of jet fuel, couldn't cause enough damage to cause the buildings to collapse. They reason they believe this is because they believe these 9/11 truth engineers when they say that the steel structures of the building should not have been damaged enough in the crashes to cause them to collapse, and that the heat caused by the burning jet fuel could not have been hot enough. The problem with this theory is that it assumes that the combination of the two should not have been able to bring down the towers because neither one on their own could have brought down the towers, and what tends to not be taken into consideration by many conspiracy theorists is while one of the two might not have been enough to bring down the towers, the combination of both was enough to bring down the towers.
Also, if you watch the videos of the collapses of the towers, it clearly shows that the beginnings of the collapses began at the impact areas. If there were any explosives on those floors, they would have gone off very shortly after the impacts, if not right at the time of impact.
Another problem with this theory is, is that it assumes that enough explosives that could have brought the towers down could have been hidden in the in the buildings for several months without being found by any of the maintenance workers, or the bomb squads and bomb sniffing dogs, who began routinely searching the building after the 1993 World Trade Center bombing.
Also, it would have been very difficult to sneak explosives into the building in the first place, as someone probably would have seen the people bringing in the explosives in the first place.
Another false assumption about this theory is, is that the towers collapsed at free fall speeds. They didn't collapse at free fall speeds. They actually collapsed for several seconds longer then what they would have if they really did collapse at free fall speeds. The posters of these videos that allegedly are claimed to shows the tower collapsing at free fall speeds either don't know what free fall speeds are, or alter the speeds of the videos to make it appear the towers were collapsing faster then they really were.
One of the biggest false assumptions is that there was explosive residue found on the wreckage at the sites. This isn't true at all, and has been independently verified.
I speculate that the reason why so many people in the 9/11 Truth movement believe that the towers were brought down by explosive is, because it would help justify their beliefs that the government was involved.