What renewable energy needs is a way to store excess energy at grid scale for later use. Gravity Power thinks they can take traditional stored hydro (pump water up to a holding tank) and do it underground. This eliminates environmental concerns with fish and they say a 7 acre site could store a massive 2 GW.
The bore hole is filled with water and sealed so it seems it would have little impact on the surrounding area and no toxic chemicals are used. This is a seriously good idea.