Theres already methods of harnessing energy from gravity (like from the flow of water), but not in very massive amounts. What if we drilled a very deep hole into the ocean (down to the extremely hot farthest reaches of the crust where water will evaporate easily) and set up a generator at the bottom? Have a massive pipe connected from the ocean water to the bottom of the pit. The hole wont fill up with water if ventilate the steam to the air above the ocean (driven by the difference in pressure). Mineral Insulated cables could be run down to the generator. I'm talking extremely hot conditions here to allow the massive amount of water flow to evaporate ASAP.. we would actually be using the earth's heat as a driving force to return the water... Law of Convservation remains unbroken.. because the steam that rises back up the hole has lost its potential energy from the deep pressures of the ocean and the gravity of the hole... and has given it to us. There's already a thread on here that talks about harnessing energy from the bottom of the ocean, but not from BENEATH the bottom of the ocean.