San Francisco
When the California Gold Rush started in 1848, San Francisco's port became very important. Immigrants came by sea from around the world. During World War II, the city was a center of war industries. In the 1990s, the economy became focused on internet companies and other high tech. Many of the world's largest companies have their main offices in San Francisco, such as Wells Fargo, Salesforce, Uber, Airbnb, Twitter, Levi's, Gap, Dropbox, and Lyft. The city has among the highest average incomes and highest housing costs in the United States.