The construction of the New York City Subway, which opened in 1904, helped bind the new city together, as did additional bridges to Brooklyn. In the 1920s Manhattan experienced large arrivals of African-Americans as part of the Great Migration from the southern United States, and the Harlem Renaissance, part of a larger boom time in the Prohibition era that included new skyscrapers competing for the skyline. New York City became the most populous city in the world in 1925, overtaking London, which had reigned for a century. Manhattan's majority white ethnic group declined from 98.7% in 1900 to 58.3% by 1990.