(Courtesy Herb Jones via Group SJR)

It is difficult, if not impossible, to imagine that there were major stories overlooked from the day of the attacks on the World Trade Center and the Pentagon.

But a new, short documentary appears to have turned up something close — the meagerly covered account of the sea evacuation of hundreds of thousands of people from Lower Manhattan, spontaneously organized by a couple of Coast Guard hands and a makeshift armada of fishing vessels, ferries and party boats. The documentary says the evacuation was the largest boatlift ever, bigger even than rescue of Allied troops at Dunkirk during World War II, when more than 300,000 British and French were evacuated from the French coast.

For nine hours, businessmen, husbands and wives, and even pets scrambled aboard boats. And for nine hours, ship captains and their crews kept returning to the docks to pick them up.

“The greatest day that I’ve seen in all my boating, my life on the water,” Herb Jones, an engineer aboard the Mary Gettaly, recalls in the film, “Boatlift.”

What made the evacuation all the more remarkable was that it took place without any formal planning, military or otherwise. Most of the vessels responded to a call from a Coast Guard lieutenant in Manhattan, Michael Day, who broadcast his request to anyone who would listen.

Narrated by Tom Hanks and directed by Eddie Rosenstein of Eyepop Productions, “Boatlift” features interviews with Day and with others who came to the rescue of civilians, estimated to number more than 500,000.

Ten years after the attacks, many of the captains and crew members say they acted as they did because they felt they had to.

“I have one theory in life. I never want to say the word I should have,” Vincent Ardolino, captain of the Amberjack V, says at the end of the film. “If I do it and I fail, I tried. If I do it and I succeed, better for me. And I tell my children the same thing: Never go through life saying you should have. If you want to do something, you do it.”

“Boatlift,” which runs just 12 minutes, is being promoted by a couple of nonprofit groups — the Center for National Policy and Voices of September 11th — which are seeking to rekindle the sense of national unity that pervaded in the aftermath of the attacks.

Along with a handful of other short films, it will be shown Thursday, Sept. 8, at an event at the Newseum to commemorate the 10-year anniversary of the attacks. It will also be available that day here.