Compressed Air Energy Storage (CAES) puts large quantities of air into an underground cavern or mine during off-peak (low electric demand) periods. At a time of day when demand is greater, this air is expanded through a turbine to generate electricity. To improve efficiency, the air may be heated with natural gas. CAES has been called "the only proven technology to store electric energy on an industrial scale of hundreds of MW’s" (http://www.espcinc.com/CAES_AI.htm)