At the end of the fourth millennium B.C., enormous mud-brick platforms had been built at a number of sites in Mesopotamia. It is presumed that they originally supported important buildings, especially temples. By the mid-third millennium B.C., some temples were being built on huge stepped platforms. These are called ziggurats in cuneiform texts. While the actual significance of these structures is unknown, Mesopotamian gods were often linked with the eastern mountains, and ziggurats may have represented their lofty homes. Around 2100 B.C., southern Mesopotamian cities came under the control of Ur-Nammu, ruler of the city of Ur. In the tradition of earlier kings, Ur-Nammu built many temples, including ziggurats at Ur, Eridu, Uruk, and Nippur. Ziggurats continued to be built throughout Mesopotamia until Persian times (ca. 500 B.C.), when new religious ideas emerged. Gradually the ziggurats decayed and the bricks were robbed for other buildings. However, their tradition survived through such stories as the Tower of Babel. By 1922, an excavation jointly sponsored by the British Museum and the University of Pennsylvania Museum under the direction of C. Leonard Woolley began at the site of Ur. In the autumn of 1923, the excavation team began to clear away the rubble around the ziggurat. Although the upper stages had not survived, Woolley used ancient descriptions and representations of ziggurats to reconstruct Ur-Nammu’s building. The Iraqi Directorate of Antiquities has since restored its lower stages.