[aaron695]

2.5 times the weight of water is nothing. It goes from it costs a shit load, to it costs a shit load / 2.5. (Assuming normal cheap water pumps still work)

Look at the diagram. The water reservoir is way bigger than 2.5 times the size, it's also way higher (So more energy). And it doesn't have to be buried.

Working in mines is a nightmare for safety, working in confined spaces is dangerous. Burying things underground is expensive.

Ponds look beautiful if done properly. They can be used by the public. You can use them as nature reserves. They are cheap, just holes in the ground.

They would be 'tidal' and you couldn't drain all the water, but to me this is the way to go. But because the project costs are simple, people won't fund them.