Around 30 million people in China live in cave homes (窰洞, yaodong). Why? Because they are warm in the winter and cool in the summer. These ones were abandoned about 1,000 years ago.
These caves were occupied about 1,100 years ago by the Xiyi people who used an early version of central heating to keep warm.
from Home Services Blog http://www.facebook.com/naturalhomes/posts/829294743790258
from Tumblr http://msalexandrakpierson.tumblr.com/post/118880081528