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Four years ago this month, archeologists monitoring the excavation of the former World Trade Center site uncovered a surprise - part of an ancient sailing ship.

At 22 feet (6.7 meters) below today's street level, in a pit that is now an underground security and parking complex, work was forced to stop as excavators uncovered the ancient ship.

Researchers were baffled by the vessel, but a new study has finally revealed how old the ship was - and how it came to be under the site.

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The entire ship was scanned before its removal to create a precise record of where each of its pieces were originally found

The team were able to analyse tree rings on the wooden 'skeleton to show the boat came from wood cut in Philadeplhia in 1773.

Tree-ring scientists at Columbia University's Lamont-Doherty Earth Observatory were among those asked to analyze its remains for clues about its age and origins.

Researchers at the lab dried the fragments slowly in a cold room and cut thick slices of the wood to get a clear look at the tree rings.

In a study now out in the journal Tree Ring Research, the scientists say 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 bloody war that established America's independence from Britain.

A team of archeologists with New York-based AKRF and others worked from dawn until dusk cataloging each timber and artifact before hauling the pieces off site.

The ship was perhaps made from the same kind of white oak trees used to build parts of Independence Hall, where the Declaration of Independence and U.S. Constitution were signed.

The key to the analysis was wood sampled from Philadelphia's Independence Hall two decades earlier by Lamont tree-ring scientist Ed Cook, according to the researchers.

Growth rings still visible in the building's timbers matched those from the World Trade Center ship, suggesting that the wood used in both structures came from the same region.

As trees grow, they record the climate in which they lived, putting on tighter rings in dry years and wider rings in wet years.

In the process, a record of the region's climate is created, allowing scientists to see how Philadelphia's climate differed hundreds of years ago from say, New York's Hudson Valley.

An array of munitions were also found on board, including a French-made cannonball and the American-made musket balls pictured.

The ship¿s frame was riddled with small holes, suggesting it had spent time in the Caribbean, a hub in the rum, sugar and slave trade at the time. Teredos, or shipworms, are native to the salty, warm waters of the South Atlantic Ocean

The climate fingerprint also serves as a kind of birth certificate, telling scientists where pieces of wood originated.

The ship itself has been tentatively identified as a Hudson River Sloop, designed by the Dutch to carry passengers and cargo over shallow, rocky water.

It was likely built in Philadelphia, a center for ship-building in Colonial times.

Several ring patterns matched oak timbers found in Philadelphia¿s Independence Hall, where the Declaration of Independence and U.S. Constitution were signed. Lamont-Doherty scientist Ed Cook sampled Independence Hall in the late 1980s while amassing his North American Drought Atlas; those records would be crucial in tracing the World Trade Center ship¿s timbers to a dense, old-growth forest near Philadelphia.

A button found aboard the ship(left) , from the coat of a private in the British Army 52nd Regiment of Foot, which battled the American rebels. On the right the rings can be clearly seen in one of the beams.

After 20 to 30 years of service, it is thought to have sailed to its final resting place in lower Manhattan, a block west of Greenwich Street.

As trade in New York harbor and the young country flourished, Manhattan's western shoreline inched westward until the ship was eventually buried by trash and other landfill.

By 1818, the ship would have vanished from view completely until the terrorist attacks on Sept. 11, 2001 set in motion the events leading to the World Trade Center's excavation and rebirth.