In general, underground water sources in the US have been discovered and are undergoing depletion.  In specific - to my knowledge - there is only one major source that hasn't been developed west of the Mississippi in the US.  That source is brackish thus requiring a good deal of energy to allow it to be used.  Outside the US?  Again, I don't really know but I suspect they have been found and are being depleted (thinking of the Aral Sea) as well wherever agriculture is currently, or has been, practiced.