If our industrial civilization is to be sustained, it must find renewable sources of energy to replace its finite and rapidly shrinking reserves of fossil carbon. Moreover, these renewables, even if intermittent, must somehow be rendered reliable and dispatchable, most probably by developing super-massive storage facilities for energy. Historically this has meant pumped hydroelectric storage, a technology that is well developed, reliable, comparatively inexpensive, and seriously limited by a shortage of suitable reservoir sites. The obvious solution is to excavate an underground reservoir many hundreds of meters below surface level and to exchange water between it and a surface reservoir created immediately above it and diked using spoil from the excavation. This variant of hydro storage is called underground pumped hydro (UPH) and is described in detail in this review, where it will be shown that: 1) the cost per GW of pumping station could be reasonable and on the order of 1 G$US while 2) the cost of storage capacity could be less than 100 $US per kWh and in keeping with the U.S. Department of Energy's cost goals.