Archaeologists believe that the first humans to enter the New World used the Bering Land Bridge as they migrated from what is now northeast Asia to northwestern North America. The "bridge" is a great land mass that emerged in the Bering and Chukchi Seas, following a drop in sea level several times in the Quaternary Period. The migration presumably occurred during a period from 30,000 to 10,000 years ago. At the end of that period, the two continents were separated by the sea as the ice in the major continental ice sheets and other glaciers melted and caused a rise in sea level.