For centuries we have been using the gravitational potential energy of water courses by building watermills and dams with hydroelectric plants. The weather does the job of bringing the water back 'upstairs' for free so conceptually that's a wonderful energy source. The problem is (I guess) that we have far too few convenient water courses to produce the required energy output. From here, several ideas arise: 1) what about building artificial convenient water courses? Study were there is a lot of precipitation on high altitude areas, in geographically convenient areas (altitude gradients etc) and build huge water lakes to be used for hydroelectric energy production (rather than just using naturally occurring water courses). 2) force / accelerate the water flow by melting the snow from very high permanently snowed mountains such as the Himalayas, for example using solar mirrors to warm up the snow. I guess that this would not alter the natural equilibrium significantly, since the cold and new snow precipitations would restore the melted snow? 3) what about using mountains and their soil / rocks instead of water? Very simply stated, imagine we build a huge ramp from the top of a mountain to the deepest point in a sensibly close distance. Then we put a bulldozer on the top of the mountain which gradually breaks down the stuff at the top of the mountain and throws it into the ramp, where it would fall and could be used similarly as water to power some kind of turbines. 4) of course that would have no natural mechanism bringing the stuff back to high altitude (unless we wait millions of years for mountain rising), but volcanoes do naturally raise 'stuff' from the Earth up. If a similar system would be placed near the top of an active volcano, just far enough from the erupting crater to avoid the whole thing getting melted, that could also be a possibility. Probably crazy?