There is a wacky conspiracy theory that claims 9/11 was executed by a group of rag tag terrorists who planned the whole operation from caves and training camps in Afghanistan.
This wild theory would have us believe that these jihadists managed to hijack 4 airliners using box cutters as weapons. Somehow, they were able to make the FAA, NORAD, and the entire U.S. military establishment unable to respond to even a single hijacking.
But wait, don't start laughing yet, there's more. They have this crazy theory that a plane full of kerosene fuel can cause a fire hot enough to turn steel molten, not only on the damaged floors, but also all the way down in the basement of the World Trade Center towers 1 and 2, where the molten steel simmered for months, releasing micro particles into the air according to a report by a Cal-Davis scientist and others. Must have been kerosene Hi-Octane!
These conspiracy nuts have attracted an avid cult following too. When anyone mentions examples of steel frame high rises around the world that have had fires that engulfed up to 23 floors and burned for over 12 hours without causing a collapse, they respond with reasoned, cogent, scientific arguments such as "You are a terrorist that hates America".
I could go on with some of the other wacky theories they have offered for the events of the day, but these theories fill a report of over 500 pages, and space here does not permit.