I recall (hopefully correctly-that was a long time ago!) a 1st yr physics project to look at a proposed 'over sum' experiment.
The system starts with water on a really tall hill, enough potential energy to electrolyse the water into hydrogen and oxygen, which are placed in balloons, the mixture I believe is lighter than air, it drags uphill, generating some electricity on the way, then at the top, you burn it back to water again. Gaining even more energy!
If I recall correctly, the 'gain' is from the atmospheric heat the balloon picks up as it ascends.