What are the spontaneous migration activities in history?

Due to the war in the Central Plains and the turn of the Song Dynasty at the end of the Han Dynasty and the Song Dynasty at the end of the Tang Dynasty, a large number of Han people moved south or moved to other frontier areas. Although the role of these immigration activities is undeniable, they are not government actions, lack of overall planning and are difficult to sustain. Because as long as it is not natural or man-made disasters or other inevitable reasons, in general, mainlanders are rarely willing to abandon their homes and leave their homes to go to frontier areas with unfamiliar living and interpersonal environment. Of course, the objective reason is that the economic model of small-scale peasant economy can rarely withstand this kind of tossing. As Zhong Changtong pointed out in Long Talk about Profit and Loss: "There are ten acres of mulberry in summer and barren fields in Yuanzhou. Secular security, if you die, you won't make it. Who can go by himself? " Therefore, after the reunification of the country, it has become an important strategy of the central government to formulate the border development strategy and mobilize the mainland people to engage in production, settlement and life in the border areas by relying on administrative resources.