Rock Springs is a city in Sweetwater County, Wyoming, United States. The population was 23,036 at the 2010 census, making it the fifth most populated city in the state of Wyoming, and the most populous city in Sweetwater County. Rock Springs is the principal city of the Rock Springs micropolitan statistical area, which has a population of 37,975. Rock Springs is known as the Home of 56 Nationalities because of the influx of immigrants from all over the world who came to work in the coal mines that supplied the fuel to power the steam engines of the Union Pacific Railroad. The city's rich cultural heritage is celebrated each summer on International Day, a festival where the foods, costumes, and traditions of residents' ancestors are recreated and enjoyed at Bunning Park in downtown Rock Springs.