Only now, as CBS News Correspondent David Martin reports, are we learning just how close the Pentagon came to having to be abandoned that day. It all happened in the first hour after the plane struck when a handful of maintenance workers saved the building from a meltdown.
Steve Carter, the assistant building manager, felt a jolt and watched the screens in his operations center turn red as hundreds of fire alarms went off at once.
It's unbelievable, Carter says, "When you sit there and you look at a whole side of a building this size basically erupt into a fire."
Over at the heating and refrigeration plant, Don Kuney saw something he couldn't believe either: The building's water pressure was plummeting.
Normally, the pressure is around 90-95 pounds. On the day of the attacks, it dropped to 18 pounds.
"OK, that's not good," says Kuney.
The plane had severed water mains and the Pentagon was losing its lifeblood. Without water there would be no way to fight the fire. Without water there would be no air conditioning in the rest of the building and the computers and electronic equipment in the military command centers would quickly overheat and crash.
Faced with an out of control fire and the threat of a massive computer meltdown, Kuney called Carter.
"My message to Steve was, 'Steve, I'm about ready to lose the plant, (and) I need somebody to get inside and shut the valves in the basement of the Pentagon,'" says Kuney.
Not wearing a breathing apparatus, Carter headed down into the basement where the smoke was getting heavier.
"It was really caustic, and it was almost choking," says Carter. "You could not draw it into your mouth."
Fighting through smoke, fighting to keep the Pentagon alive, Carter reached the large valves.
"At the same time you're sitting there trying to breathe through the armpit of your coat and operate the valves," he says.
You think about the huge building, the largest office building in the world and all of that, and saving it comes down to the one valve in the basement.
"Without closing these valves, water continues to pour," Carter says. "The fire fighting water can't build up pressure … and all of the (building's) electronics would be gone."
The race to save the water had been won, but the battle to save the Pentagon was not over.
In Part 2 of Martin's report, learn how Steve Carter and his blue-collar warriors kept the fire from forcing the military to abandon its headquarters.