Bright Energy Storage Technologies is developing an ultra-low-cost underwater energy storage system. Energy from the electric grid, or from an offshore renewable energy source, compresses air, stores it in vessels at the bottom of a body of water, and then generates electricity when released back to the grid. Bright Energy storage systems yield dramatic reductions in the net cost and availability of grid-scale energy storage. They can be deployed offshore, in lakes, and in many reservoirs. Storing air at hydrostatic pressure, in equilibrium with the surrounding water allows the use of thin-walled storage vessels, and simpler, more reliable compressors. A simple, proprietary reversible air compressor runs at near-constant temperature and uses surrounding water as a heat sink and source, eliminating the need for fossil fuel.