In the United States, the hull of a ship likely made in the 1700s was discovered at the World Trade Center site. Archaeologists say that it probably was sunk there in the effort to add land to Manhattan in the early 19th century. Molly McDonald, a US archaeologist, 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. 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.