"An idea whose time has come" is an apt description of underground construction, in 2000. It is of course an idea as old as the hills, - or, at least, the caves in the hills - and energy conservation is just one of a number of important advantages which it boasts. But energy conservation is very important to the United States and other industrialized nations in 2000, and so we should draw attention to the part that underground construction could play in the effort to conserve fuel. The potential for energy conservation by underground construction is therefore very large. It must also be emphasized that energy conservation, as distinct from the discovery of new sources of energy, must become much more a way of life for all of us. As the Office of Technology Assessment of the U.S. Congress points out "...successful wide-spread implementation of conservation programs with increased efficiency or waste reduction objectives can have both a rapid and a continuing effect. Such improvements need not be technologically complex. ...If ERDA is to provide near-term and mid-term energy problem solutions, conservation through efficiency and waste-reduction programs should be essential ingredient." Increasing prices for fuel are forcing us to adopt this approach but the full magnitude of the problem does not appear to be as widely recognized, as it should be. It is simply that continuation of the present rates of increase fuel consumption would eventually result in no fuel at any price.