The Prehistory of Siberia is marked by several archaeologically distinct cultures. In the Chalcolithic, the cultures of western and southern Siberia were pastoralists, while the eastern taiga and the tundra were dominated by hunter-gatherers until the Late Middle Ages and even beyond. Substantial changes in society, economics and art indicate the development of nomadism in the Central Asian steppes in the first millennium BC. History of research Scholarly research of the archaeological background of the region between the Urals and the Pacific began in the reign of Peter the Great (1682-1725), who ordered the collection of Scythian gold hoards and thereby rescued the contents of several robbed graves before they were melted down. During his reign, several expeditions were charged with the scientific, anthropological and linguistic research of Siberia, including the Second Kamchatka Expedition of the Dane Vitus Bering (1733-1743). Scholars also took an interest in archaeology and carried out the first archaeological excavations of Siberian kurgans. After a temporary reduction of interest in the first half of the nineteenth century, archaeological research in Siberia reached new heights in the late nineteenth century. Excavations were particularly intense in South Siberia and Central Asia. The results of the October Revolution 1917 created different, often restricted, conditions for archaeological research, but led to even larger projects, especially rescue excavations as a result of gigantic building projects. Eventually, even remote areas of the Soviet Union such as Sakha and Chukotka, were archaeologically explored. After the Second World War, these developments continued. Following the Collapse of the Soviet Union in 1991, much more intensive collaboration with the west became possible. Topography Siberia is characterised by a great deal of variety in climate, vegetation, and landscape. In the west, Siberia is bordered by the Ural Mountains. From there, the west Siberian lowlands extend to the east, all the way to the river Yenisei. Beyond this are the central Siberian highlands which are bordered on the east by the basin of the Lena River, beyond that are the northeast Siberian highlands. Siberia is bordered on the south by a rough chain of mountains and to the southwest by the hills of the Kazakh border. The climate in Siberia is very variable. Yakutia, northeast of the Lena, is among the coldest places on Earth, but every year temperatures may vary for more than 50 °C, from as low as −50 °C in winter to over +20 °C