into the Middle East, South Asia, and the borders of China, where the Tocharians resided. The northernmost part of Asia, including much of Siberia, was largely inaccessible to the steppe nomads, owing to the dense forests, climate and tundra. These areas remained very sparsely populated. The center and the peripheries were mostly kept separated by mountains and deserts. The Caucasus and Himalaya mountains and the Karakum and Gobi deserts formed barriers that the steppe horsemen could cross only with difficulty. While the urban city dwellers were more advanced technologically and socially, in many cases they could do little in a military aspect to defend against the mounted hordes of the steppe. However, the lowlands did not have enough open grasslands to support a large horsebound force; for this and other reasons, the nomads who conquered states in China, India, and the Middle East often found themselves adapting to the local, more affluent societies. The Islamic Caliphate's defeats of the Byzantine and Persian empires led to West Asia and southern parts of Central Asia and western parts of South Asia under its control during its conquests of the 7th century. The Mongol Empire conquered a large part of Asia in the 13th century, an area extending from China to Europe. Before the Mongol invasion, Song dynasty reportedly had approximately 120 million citizens; the 1300 census which followed the invasion reported roughly 60 million people. The Black Death, one of the most devastating pandemics in human history, is thought to have originated in the arid plains of central Asia, where it then travelled along the Silk Road. The Russian Empire began to expand into Asia from the 17th century, and would eventually take control of all of Siberia and most of Central Asia by the end of the 19th century. The Ottoman Empire controlled Anatolia, most of the Middle East, North Africa and the Balkans from the mid 16th century onwards. In the 17th century, the Manchu conquered China and established the Qing dynasty. The Islamic Mughal Empire and the Hindu Maratha Empire controlled much of India in the 16th and 18th centuries respectively. The Empire of Japan controlled most of East Asia and much of Southeast Asia, New Guinea and the Pacific islands until the end of World War II. Geography Asia is the largest continent on Earth. It covers 9% of the Earth's total surface area (or 30% of its