The most decorated heroes of 9/11 were the ones on the ground – firefighters, police, volunteers – who saved lives and in many cases lost their own in the process.

But a lesser-known effort took place on the water, as leaving lower Manhattan by familiar routes proved all but impossible after the attacks that took place 20 years ago.

James Kendra and Tricia Wachtendorf, from the University of Delaware's Disaster Research Center, can tell the story of how hundreds of boat operators and their crews helped people escape the island via New York Harbor.

Kendra and Wachtendorf chronicled the effort in their book, "American Dunkirk: The Waterborne Evacuation of Manhattan on 9/11."

The two social scientists arrived in New York on Sept. 13, 2001, and ended up staying two months as they conducted interviews and delved into various aspects of the response to the terrorist attacks. They returned about a year later for follow-up research.