A friend has asked a question of me that has me wishing that my days of thermodynamics were not so long in the past. He and some colleagues have been making electrolytic cells for high production of hydrogen and oxygen from water. In demonstrating one to me he posed an experiment that on the surface seems to produce free energy, which we all know is impossible. They have tested the cell under pressure and found that it's gas production and the amount of electricity used are no different than at normal atmospheric pressure. He proposed the following setup. Place the electrolytic cell 30 meters under the water (or what ever, the depth does not matter). Flip the switch and hydrogen and oxygen are produced , no surprise there. The energy content of the gasses is not the issue. The gasses can also provide work when they rise to the surface. The amount of gas produced and the amount of electricity used depth is the same as produced at the surface so that part of the equation is a wash. The work provided by placing the cell beneath water and harnessing energy from the rising gas is the part that I cannot account for. What am I missing???