On every continent, diverse renewable resources could supply all humanity's future energy needs. Earth's richest renewable energy resources-large in geographic extent, high intensity-are usually stranded: far from end-users without gathering and transmission systems. The output of most renewables varies greatly, at time scales of seconds to seasons: the energy capture assets thus operate at inherently low capacity factor (CF), and energy delivery is not 'firm'. New electric transmission systems dedicated to renewables will suffer the same low CF and represent substantial stranded capital assets. At gigawatt (GW) scale, renewable-source electricity could be converted to hydrogen and oxygen, via high-pressure-output electrolyzers. Hydrogen would be pipelined to load centers for use as vehicle fuel, retail-value combined-heat-and-power generation on the customers' side of meters, ammonia production, and refinery feedstock. Great Plains, U.S.A. wind energy alone, if fully harvested and pipelined to markets, could supply all U.S.A. energy. About 15 000 new, large, solution-mined Great Plains salt caverns could economically store enough energy as compressed hydrogen to 'firm' this wind power at annual scale, at an incremental capital cost to the generation-transmission system of 5-10%. Worldwide, firming in geologic formations adds great market and strategic value to diverse, stranded, and abundant renewables. Copyright (C) 2008 John Wiley & Sons, Ltd.