Completed in 1959, the 183-mile (294-kilometer)-long St. Lawrence Seaway was built by deepening and widening the St. Lawrence River between Montreal, Canada, and Lake Ontario so that large ships could traverse it. The Seaway consists of a series of locks that allow ships to travel from the Atlantic Ocean to the Great Lakes, and ultimately on to Chicago. A limiting factor of the Seaway is that ships can only use the Seaway between May and November, as it is blocked by ice in the winter.