one of the largest Union Army hospitals of the Civil War, operated from 1862 to 1865; part of its grounds are now the northern section of Clark Park. West Philadelphia's population expanded in the late 19th and early 20th centuries, thanks in large part to horsecars, then streetcars, and Schuylkill River bridges that allowed middle-class breadwinners to commute into the Central Business District a few miles to the east. West Philadelphia was among the early streetcar suburbs, and a portion of it is listed on the National Register of Historic Places as the West Philadelphia Streetcar Suburb Historic District. The western portion of the neighborhood was once home to some of the most expensive real estate in the country (and much striking Victorian-era architecture remains). The area has declined in prominence over the last 50 years, thanks in part to increasing crime and the migration of many middle and upper-class residents to suburbs and other sections of the city. West Philadelphia drew national attention in 1978 and 1985 for violent clashes between police and an Afro-centric, back-to-nature group called MOVE. During the latter confrontation, police firebombed the group's headquarters, killing 11 people and destroying an entire block of Osage Avenue and Pine Street. In recent years, parts of West Philadelphia have undergone "Penntrification," a term that reflects the University of Pennsylvania's role in gentrification of the neighborhood; including a campaign to rename part of the area University City (the designation now appears on maps as well as public street signs and government listings). Many young professionals and families have moved into the area. In 2008, the area around the Please Touch Museum, Philadelphia Zoo, and the Mann Music Center was designated the Centennial District: an area to be revitalized by the country's 250th birthday in 2026. Housing Most of the houses in West Philadelphia are row houses, although there are areas of semi-detached and detached houses. The earliest developments began in 1850 and the final period of mass construction ended in 1930. Development was enabled by the creation of the horsecar, which pushed development to about 43rd Street, and, after the arrival of the electrified streetcar in 1892, accelerated to the west and southwest. Largely commissioned by speculative developers and designed by some of the city's most prolific architects, they were purchased by industrial managers and other professionals who led the first movement of upper and middle class from the