Six years ago, archaeologists excavating the former World Trade Center site uncovered a surprise that forced them to halt their work.
They had uncovered part of an ancient sailing ship 22ft below street level, in a pit that is now an underground security and parking complex.
Researchers were baffled by the vessel but a study released two years ago revealed how old the ship was and how it had got there.
Video credit: University of Maine
Tree-ring scientists at Columbia University's Lamont-Doherty Earth Observatory analysed the tree rings on the wooden skeleton to show the boat came from wood cut in Philadelphia in 1773.
"What determined the exactness of the construction date of the ship was the use of [tree ring dating] which was made possible by recovering timber from the ship," said Cook, a lead researcher on the project. "It really is the premier scientific method for dating structures made of wood."
Researchers at the lab dried the fragments in a cold room and cut slices of the wood to get a clear view of the tree rings.
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In a study in the journal Tree Ring Research, the scientists said that an old growth forest in the Philadelphia area supplied the white oak used in the ship's frame, and that the trees were probably cut in 1773 or so - a few years before the War of Independence.
It is thought to have sailed to its final resting place in lower Manhattan, a block west of Greenwich Street after 30 years of service as a trading ship. But as trade in New York harbour flourished, Manhattan's shoreline moved westward until the ship was eventually buried by trash and other landfill.
It is thought the ship would have vanished from view by 1818, only to be uncovered following the 9/11 attacks.
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