Harbor Island is an artificial island in the mouth of the Duwamish River in Seattle, Washington, US, where it empties into Elliott Bay. Built by the Puget Sound Bridge and Dredging Company, it was completed in 1909 and was then the largest artificial island in the world, at 350 acres . Since 1912, the island has been used for commercial and industrial activities including secondary lead smelting, shipbuilding and repair, bulk petroleum storage, metal fabrication and containerized cargo shipping. Warehouses, laboratories and other buildings are located on the island.