Surprise is a city in Maricopa County, Arizona. The population was 30,848 at the 2000 census; however, rapid expansion has boosted the city's population to 117,517 at the 2010 census, an increase of 281%. As such, it is the second fastest-expanding municipality in the greater Phoenix metropolitan area and, between 1990 and 2000, it was the sixth fastest-expanding place among all cities and towns in Arizona.The city has a 10,562-square-foot Aquatics Center and Maricopa County's northwest regional library, a $5.5 million, 20,000-square-foot library, along with a 100.3 cost of living index.