Somehow, there are still people who don’t believe that on Sept. 11, 2001, a group of terrorists seized four commercial jetliners and piloted them toward New York and Washington, killing thousands of people.

Never mind the reams of sober and professional reports that have explored what actually happened and why. Instead, a cottage industry of myths has sprung up about that terrible day, such as:

* that a missile struck the Pentagon, not a Boeing 757

* that U.S. air defenses were ordered to “stand down” on 9/11

* that government agents gathered all of the passengers from the four jets on Flight 93, and then brought it down

* that World Trade Center 7 was professionally demolished.

A team of journalists from Popular Mechanics has done a remarkable job exploring 25 of the most prominent conspiracy theories. Their findings have been recently published in an updated edition of the book “Debunking 9/11 Myths,” edited by David Dunbar and Brad Reagan.

The book completely demolishes all of the most outlandish assertions, using actual facts and information provided by an impressive array of experts. It is well worth reading. As a reader service, we provide a summary of Popular Mechanics’ findings on some of the most persistent myths.

The Air Force was ordered to stand down on 9/11, allowing the plot to unfold

The Facts:

Only 14 fighter jets were on alert in the contiguous 48 states. At the time, there was no computer network or alarm system in place to warn defense command of missing civilian planes. The system in place assumed that hijackers would make demands, not turn off transponders and destroy jets. There was no order to stand down.

The Empire State Building did not collapse when a B-25 bomber struck it in 1945, so the World Trade towers should have remained standing.

The Facts:

The Empire State Building was made of reinforced-concrete columns and a thick masonry exterior — a density of 38 pounds per cubic foot. The World Trade Center, built in 1970, was designed in a completely different way, with the load-bearing core in the center, surrounding by thin exterior shell — a density of eight to nine pounds per cubic foot. That’s lighter than balsa wood.

The fire from jet fuel was never hot enough to melt steel, so the towers shouldn’t have fallen.

The Facts:

The fires did not need to get hot enough to melt steel, but that’s unimportant. Instead, the steel frame just needed to lose some of its structural strength for collapse to occur. The heat rose to as much as 1,000 degrees Celsius (1,832 Fahrenheit), at which point steel loses 90 percent of its strength.

The steel trusses had spray-on fireproofing that crumbled from the touch of a hand, and the jet fuel ignited rugs, curtains, furniture and paper, creating an inferno that lasted longer than the jet fuel fire.

“Nano-thermite” was found in the towers, providing evidence of a controlled demolition

The Facts:

The material cited by conspiracy theorists is aluminum alloys from the jet planes. Tons of thermite would have been required to cause the extensive column damage in World Trade Center tower 2.

World Trade Center 7 collapsed, even though no plane hit it, because the government destroyed it with pre-planned explosions.

The Facts:

The building collapsed because of long-burning fires in its interior, which had started from debris damage caused by the collapse of Tower 1. The fires began as small, single cubicle fires on 10 different floors, and then adjacent cubicles caught fire, superheating offices that then burst into flame.

The density of paper materials in the office building exacerbated the situation. Meanwhile, the water supplies for the building’s sprinkler system were eliminated by the collapse of the Twin Towers.

A missile struck the Pentagon, not a plane

The Facts:

Hundreds of commuters saw the plane crash into the Pentagon and at least two people on board the plane phoned family members to tell them the plane had been hijacked.

While conspiracy theorists claim that the holes in the building were too small to be made by a Boeing 757 jet, the wings on the jet were sheared off before striking the Pentagon façade. Since the wings hold much of the plane’s fuel, at least one fifth of the fuel never entered the building.

Moreover, because of the Pentagon’s reinforced concrete columns and the energy load of the plane, the jet’s exterior crumbled upon impact. The plane flowed into the building more as a liquid than a solid mass. Finally, pieces of the plane’s fuselage were found on the Pentagon lawn.

Flight 93 was shot down

The Facts:

Records show that there was no shoot-down order in effect for Flight 93 because of garbled communications between agencies. When the plane crashed, the defense command was still unaware it had been hijacked.

The number of passengers on all of the jets exceeds the number that could fit on a Boeing 757 (Flight 93), rendering moot the claim that all of the passengers were herded onto it before it was shot down.

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