Silver Spring is an unincorporated area and census-designated place (CDP) in Montgomery County, Maryland, United States. It had a population of 71,452 at the 2010 census, making it the fourth most populous place in Maryland, after Baltimore, Columbia, and Germantown. The urbanized, oldest, and southernmost part of Silver Spring is a major business hub that lies at the north apex of Washington, D.C. As of 2004, the Central Business District (CBD) held 7,254,729 square feet (673,986 m2) of office space, 5216 dwelling units and 17.6 acres (71,000 m2) of parkland. The population density of this CBD area of Silver Spring was 15,600 per square mile all within 360 acres (1.5 km2) and approximately 2.5 square miles (6 km2) in the CBD/downtown area. The community has recently undergone a significant renaissance, with the addition of major retail, residential, and office developments. Silver Spring takes its name from a mica-flecked spring discovered there in 1840 by Francis Preston Blair, who subsequently bought much of the surrounding land. Acorn Park, tucked away in an area of south Silver Spring away from the main downtown area, is believed to be the site of the original spring.