Seattle is the largest metropolitan area in the northwestern quarter of the United States, and it played host to the 1962 World's Fair. The city is crowded onto an isthmus, with suburbs to the north, east, and south of Lake Washington, connected by two bridges. Settled in 1851, the city was laid out two years later as the seat of King County. It served primarily as a lumber center until the 1880s, when the arrival of the railroad spurred economic development. The Yukon gold rush made Seattle an important commercial center, as did the opening of the Panama Canal. A major shipbuilding center during both world wars, Seattle experienced a boom from aircraft manufacturing during World War II.