The Jin dynasty, founded by Jurchen peoples from Manchuria, captured northern China from the Liao dynasty and needed a capital that projected the authority of conquerors. They chose the site of the Liao dynasty's southern capital, Nanjing, and built over it on a grander scale. Zhongdu became the Jin capital in 1153 and grew into a metropolis measuring roughly three to four miles across. By the late 12th century, its population approached one million -- a staggering number for any medieval city and evidence of the Jin's ambition to rival the Southern Song dynasty to the south. Among the various capitals built on the site of modern Beijing over the centuries -- Liao Nanjing, Jin Zhongdu, Yuan Dadu, and Ming-Qing Beijing -- Zhongdu was the second smallest, but in its time it was a center of administration, commerce, and culture.

In 1215, the Mongol Empire's armies swept down from the steppe and destroyed Zhongdu. The siege and sack were thorough. The city that had housed nearly a million people was reduced to ruins, its walls breached, its population scattered or killed. The destruction was so complete that when the Mongols established the Yuan dynasty and Kublai Khan decided he needed a proper capital, he did not rebuild on the same site. Instead, he ordered construction of an entirely new city -- Dadu, known in the West as Khanbaliq -- to the northeast of the ruined Zhongdu. The decision to build adjacent rather than on top of the old capital was unusual and suggests the extent of the devastation.

The layers of Beijing's history stack vertically. Liao Nanjing lies deepest, then Zhongdu, then Dadu to the northeast, and finally the Ming and Qing capital that became the city we know today. Each iteration shifted the urban center slightly, but all occupied roughly the same patch of the North China Plain. Zhongdu's footprint in southwestern Xicheng District is now covered by modern streets, apartment blocks, and the dense fabric of a city that has been continuously inhabited for over three thousand years in various forms. Occasional archaeological excavations reveal fragments -- foundations, pottery, tools -- of the Jin-era city below. But unlike the Forbidden City or the Temple of Heaven, Zhongdu offers nothing to see from the surface. Its story must be told, because it cannot be shown.