 Archaeological information about the earliest historical rulers of Japan may lie within the ancient tombs known as kofun, constructed between the early 3rd century and the early 7th century AD.However, since the Meiji period, the Imperial Household Agency has refused to open the kofun to the public nor to archaeologists, citing their desire not to disturb the spirits of the past Emperors.In December 2006, the Imperial Household Agency reversed its position and decided to allow researchers to enter some of the kofun with no restrictions.