Beyond the 2nd Ring Road, the city spreads out in a checkerboard fashion marked by concentric ring roads. Four other districts, Haidian, Chaoyang, Fengtai and Shijingshan, were once considered on the city's outskirts, but are now integral parts of the city inside the 5th Ring Road. All four have population densities above 4500 persons/km2 (11,600 persons/km2). The vast Haidian District to the northwest is home to the Old and New Summer Palace, the university district, and Zhongguancun, the city's high-technology silicon village. The even bigger Chaoyang District embraces the city from the north and east. Its possessions include Beijing Central Business District (CBD), the Olympic Green, and even the Beijing Capital International Airport. Shijingshan is a heavily industrialized district in the foothills of the Western Hills. Fengtai spans across the south of urban Beijing.