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I have always assumed that this was mainly due to the earth acting as a
very large thermal resevoir, averaging the daily and seasonal temperature
variations. Now, I'm not so sure. Are caves in the tropics cool, or near
the average air temperature? How far down does the permafrost extend in
Antarctica?
Permafrost underlies an estimated 20%-25% of the world's land surface;
it occurs in more than 50% of Russia and Canada, 82% of Alaska, 20% of
China, and probably all of Antarctica. Permafrost in northern Siberia is
1,600 meters, (5,250 feet) thick and it is 650 meters (2,100 feet) thick
in northern Alaska.