Silver Spring is an urbanized, unincorporated area in Montgomery County, Maryland, United States. After Baltimore and Columbia, the Silver Spring CDP is the third most populous place in Maryland. 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 m) of office space, 5216 dwelling units and 17.6 acres (71,000 m) of parkland. The population density of this CBD area of Silver Spring was 15,600 per square mile all within 360 acres (1.5 km) and approximately 2½ square miles 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.