Jack Holmes in Esquire:
An extraordinary event requires an extraordinary explanation. But for some, the idea that 19 men could commandeer four commercial airliners in a coordinated attack and use them as 400-ton missiles to destroy such massive buildings still doesn't make sense. Even 15 years after the fact, there are still plenty who cannot believe that these symbols of American power—military, economic, and, had they not been stopped, political—were so fundamentally vulnerable to destruction. People want more, and when the official accounts aren't satisfying, they begin to look elsewhere.
There are still plenty of 9/11 conspiracy theorists, and no matter what you tell them, they will tell you the attacks on September 11, 2001 did not happen the way our government or our media claim. Conspiracy theorists will tell you it was an inside job. They will tell you the government let it happen. They will tell you the buildings couldn't have fallen that way, or the Air Force could have stopped the whole thing if they wanted to.
There are an untold number of theories about what really happened that day. They are the subject of a well-known documentary film, and they have spawned countless websites. Most of them are weird, and some are almost comical. But exploring these theories involves venturing into the darkest corners of our imagination, where confusion and despair bleed into reactionary paranoia, and the enduring sacrifice of the innocent people who died that day is almost cheapened by slander and suspicion.
Below are nine of the most prominent theories, as well as the evidence explaining why they simply don't add up.