Trade began to revive under the Tang dynasty. The Silk Road was renewed and thrived as caravans carried goods between China and Southwest Asia and South Asia. The Silk Road was more than just a trading route, however. It was also a conduit of ideas. Three religions==Buddhism, Christianity and Islam-spread along the Silk Road. Technical knowledge from China-including the secrets of printing, drilling wells, and making iron, gunpowder, paper and silk-reached the West along the Silk Road. Trade with regions near China, such as Japan and Korea, also increased during the Tan and Song dynasties. The Chinese exported tea, silk and porcelain to the countries beyond the South China Sea. In return, they received exotic woods, precious stones, and various tropical goods. As a result of trade, Changan, with a population of about two million, became the wealthiest city in the world.