The Bering land bridge joined present-day Alaska and eastern Siberia at times during the Pleistocene ice ages. 

Its greatest extension was about 1,000 miles (1,600 km) north to south.  It was not glaciated because snowfall was extremely light as the winds from the Pacific Ocean lost their moisture over the fully glaciated nearby mountains.  

The grassland steppe, including the land bridge, that stretched for several hundred miles into the continents on either side has been called Beringia. It is believed that a small human population of at most a few thousand survived the ice age in Beringia. It was isolated from  populations in Asia for at least 5,000 years. Sometime after 16,500 years ago, it started to populate the Americas when the American glaciers that blocked the way southward melted.

Geography 
 

The Bering Strait, the Chukchi Sea to the north and the Bering Sea to the south, are all shallow seas (maps, right).  During cycles of global cooling, such as the most recent ice age, much water became concentrated in the ice caps of the Arctic and Antarctica. The  drop in sea levels exposed shallow sea floors that have subsequently re-flooded.  Other land bridges around the world have been created and re-flooded in the same way: about 14,000 years ago, mainland Australia was linked to both New Guinea and Tasmania. The British Isles were an extension of continental Europe via the dry bed of the English Channel; and the dry basin of the South China Sea linked Sumatra, Java and Borneo to the Asian mainland.

The rise and fall of global sea levels exposed and submerged the bridging land mass called "Beringia" several times in the Pleistocene. The Beringian land bridge is believed to have existed in the glaciation that occurred before 35,000 BP.  During the more recent period 22,000-7,000 years BP it was also above water. The strait reopened about 15,500 BP and by c. 6000 BP the coastlines had their present form.

Human habitation 
The Beringa land bridge is significant for several reasons, not least because it is believed to have enabled human migration to the Americas from Asia about 25,000 years ago.  A study by Hey has indicated that of the people migrating across this land bridge during that time period, only 70 left their genetic print in modern descendants.

Further reading
 Pielou E. C. 1992. After the Ice Age : the return of life to glaciated North America (Chicago: University of Chicago Press)
Hey, Jody 2005. On the number of New World founders: a population genetic portrait of the peopling of the Americas. In Public Library of Science Biology. 24;3(6):e193

References

Other websites 
 Bering Land Bridge National Preserve
 Beringian Heritage International Park Program
 D.K. Jordan, "Prehistoric Beringia"
  Paleoenvironmental atlas of Beringia: includes animation showing the gradual disappearance of the Bering land bridge
 Yukon Beringia Interpretive Centre
 Paleoenvironments and Glaciation in Beringia 
Study suggests 20000 year hiatus in Beringia
Mitochondrial Population Genomics Supports a Single Pre-Clovis Origin with a Coastal Route for the Peopling of the Americas , American Journal of Human Genetics, 82 (3), 583-592 (3 March 2008). pdf file of article (525kb)

Geology
Arctic