New York (CNN) -- The hull of a ship likely made in the 1700s was discovered at the World Trade Center site Tuesday. Archaeologists say that it probably was sunk there in the effort to add land to Manhattan in the early 19th century.

Molly McDonald, an archaeologist with the firm AKRF, said that about one-third of lower Manhattan is man-made, constructed sometime between 1797 and 1836. Prior to 1797 the site of the ship was part of the Hudson River, as was about half of the ground zero site.

McDonald said that timber crisps and houses were sunk into the ground to create piers and cliffs.

"This ship was most likely sunk for the same purpose, to retain land, to create new land," she said.

A handful of ships have been found in the past 40 years, she said, "but it's quite unusual and exciting to find it here on the World Trade Center site."

Doug Mackey, the chief regional archaeologist for the New York State Historic Preservation Office, said that finding the ship is very exciting. "We've known that those things exist but we don't get the opportunity to record them very often. It is a unique experience."

The discovery highlights a curious aspect of Manhattan: The land is so valuable that government officials and landowners have searched for ways to add more, Mackey said.

The ship was found just south of where the World Trade Center towers used to stand, about 20 feet below the surface, It was not touched by earlier construction. McDonald and her AKRF colleagues Elizabeth Meade and A. Michael Pappalardo have been monitoring the site for over a year.

They found historical maps and documents suggesting that excavation in Lower Manhattan would reveal landfills that might hold unusual artifacts. Early Tuesday morning their work paid off when McDonald spotted the curved timber of the ship poking through the soil and mud.

The environment in the landfills was not suitable for organisms that normally break down wood over time. Now that it has been uncovered, the ship will no longer be preserved, so it is up to archaeologists to document and study the vessel before it has deteriorated.

"Right now we're documenting it as best we can," Meade said. "We're making very careful drawings, we're taking photographs. We want to learn everything we possibly can about it right now and then we'll analyze it later."