"The general idea is to have a closed vessel sitting on the seafloor. Surplus energy is then used to pump water out of this vessel, leaving the inside at a near-vacuum. When it’s desired to recover energy from the system, water can be allowed to flow back into the vessel under the pressure generated by the seawater above. As the vessel is filled, the water flowing in turns a turbine, generating electricity in just the same way as a traditional pumped hydro system."
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https://hackaday.com/2022/02/02/underwater-tanks-turn-energy...
https://www.iee.fraunhofer.de/en/topics/stensea.html
That's an interesting way to store energy, basically a vacuum chamber that operates in 75 atm water, instead of 1 atm air.