Building DD1 is a contributing element of the Mare Island Historic District. Building DD1 (Dry Dock 1) is arguably the most important single structure at Mare Island, whether considered from the historical or architectural vantage point. Functionally, the structure was the raison d'etre of the 19th century shipyard. From the standpoint of historic engineering, it is simply one of the most remarkable examples of stone masonry construction anywhere in the United States. It took nearly two decades to complete this massive granite block structure, reflecting to some degree the vagaries of funding for the project but to a larger degree the enormous challenge of the construction project. Standing almost completely unmodified from its appearance in 1891, Dry Dock 1 is at the heart of the historic district at Mare Island. - US Library of Congress