Located in the marshes of southern Mesopotamia, Nippur was one of the most ancient and most sacred Sumerian cities. To the explorers of the nineteenth century, the site offered tantalizing clues to the early history of civilization. Penn began a decade of excavation in 1889, revealing a multi-layer site with a long and complicated history. Interested in the deep, ancient Sumerian levels, the excavators failed to fully understand the upper levels. The lower layers had been severely disrupted by monumental constructions by the Parthians, an empire that flourished in the first and second centuries BCE and CE, while layers above can be dated up to the eighth c. CE.Sorting out the complex chronology of Nippur was no easy task. The earliest settlement may be as old as 5000 BCE, but the city flourished from about 3000 BCE onward. In the period known as Ur III (2200-2100 BCE), the city covered an area of 135 hectares (334 acres). The site shrank considerably at the end of the eighteenth century BC, but was never completely abandoned. A fortified Ekur continued to exist. It was then revitalized for two centuries by the Kassites before declining around 1155 BCE. After 800 BCE it came under Assyrian domination followed by the Neo-Babylonians, Seleucids, Parthians, Sasanians, and early Muslim dynasties.