Many vestiges of human habitation from the late Palaeolithic and early Mesolithic ages can be found in Hanoi. In 1971–1972, archaeologists in Ba Vì and Đông Anh discovered pebbles with traces of carving and processing by human hands that are relics of Sơn Vi Culture, dating from 10,000 to 20,000 years ago. In 1998–1999, the Museum of Vietnamese History (now National Museum of Vietnamese History) carried out the archaeological studies in the north of Dong Mo Lake (Son Tay, Hanoi), finding various relics and objects belonging to Sơn Vi Culture – in the Paleolithic Age, 20,000 years ago. During the mid-Holocene transgression, the sea level rose and immersed low-lying areas; geological data clearly show the coastline was inundated and was located near present-day Hanoi, as is apparent from the absence of Neolithic sites across most of the Bac Bo region. Consequently, from about ten thousand years to approximately 4,000 years ago, Hanoi in general was completely absent. It is believed that the region has been continuously inhabited for the last 4,000 years.