Workers excavating at the site of the World Trade Center came across a sunken ship, buried deep underground, that experts think was built in the 18th century. Originally, Manhattan Island was about two thirds the size that it is now, and the extra third was constructed during the 18th and 19th centuries. Apparently the builders of the island sunk this ship to act as part of the man made reef that would act as an extension on the island. The ship is about 30 feet long and was discovered about 20-30 feet below street level. Back in the 18th century, this site would have been near where two wharfs once stood. The New York Times has a pretty good slideshow of the ship, along with their article here: