In 1903 C.E., Panama and the United States signed a treaty by which the United States undertook to construct an interoceanic ship canal across the Isthmus of Panama, connecting the Pacific to the Atlantic. The monumental project was completed in ten years at a cost of about $387 million. The building of the Panama Canal involved several unique problems in both engineering and sanitation. The engineering problems involved digging through the Continental Divide; constructing the largest earth dam ever built up to that time; designing and building the most massive canal locks ever envisioned; constructing the largest gates ever swung; and solving environmental problems of enormous proportions. The sanitation problems included influenza (probably the major scourge of the world at the time), yellow fever, and malaria, as well as simply cleaning up after so many workers and feeding them properly. Completed in 1913 C.E., by treaty the Canal was turned back over to the nation of Panama in 1997 C.E., although the U.S. retained favored use of it.