The Kofun era is named after the great burial mounds which were tombs for the political elite. They are considered by many as the pyramids of Japan. It was a period defined by social stratification and the rise of the Yamato kingdom, which managed to conquer most of western and central Japan. The Ainu were pushed further into northern Honshu and Hokkaido. Shinto became a state religion, enforcing the rule of the kings. Technology in infrastructure, agriculture and weapons evolved with the help of constant trade with the three Korean kingdoms and the Chinese dynasties. Archaeologists and historians are still working to separate myths from real events and, with the help of cutting edge technology, are still coming up with compelling new theories.