Port Seattle, Washington.
Washington’s Harbor Island is the largest man-made island in the United States. Located at the mouth of Seattle’s Duwamish Waterway that empties into Elliott Bay, the island was built by the Puget Sound Bridge and Dredging Company in 1909. The island was built from 24 million cubic yards of earth, including dredge spoils from the Duwamish River and soil from regrades of Jackson Hill and Dearborn Street. At 350 acres, it was the world’s largest artificial island when it was built.
Filling in the vast tide flats South of downtown Seattle, the dredging also created the area now occupied by Union Station, King Street Station, Seattle’s sports stadiums, and a significant portion of the city’s industrial base.
Growing resentment of railroad and shipping monopolies led King County voters to create the Port of Seattle in 1911, and Harbor Island became part of it.
Harbor Island lost its title as the world’s largest man-made island in 1938 when Treasure Island in the San Francisco Bay was completed. However, in 1967, Harbor Island was increased by adding fill to 395 acres and regained its title. Although it remains the largest artificial island in the United States, the Rokko Island in Japan is now more than three times larger.
After World War II, the Port of Seattle diversified its activities by expanding the Fishermen’s Terminal, constructing Shilshole Bay Marina, undertaking ambitious developments on Harbor Island, and creating Seattle-Tacoma International Airport.
Over the years, the island has been used for commercial and industrial activities, including shipbuilding and repair, lead smelting, petroleum storage, metal fabrication, and containerized cargo shipping.
Fringed by wharves and cranes, the island houses numerous warehouses, laboratories, other industrial buildings, and railroad yards. It is the Port of Seattle’s primary point of entry for cargo transferred from oceangoing vessels to trucks and railcars.
Fishermen utilize the waters surrounding Harbor Island to catch shellfish and crabs.
No residences are on Harbor Island, but about 10,000 people live within one mile of the site.
Both the West Seattle Bridge and the Spokane Street Bridge pass over the island.
©Kathy Alexander/Legends of America, updated October 2026.
Also See:
Ghost Stories Across the Nation
History & Hauntings of Pike Place Market in Seattle
Washington – The Evergreen State
Washington Fun Facts and Trivia
See Sources.