That doesn't match the described mechanism at all. The load on the concrete is absolutely related to depth, and immense. They describe one option with a connect

That doesn't match the described mechanism at all. The load on the concrete is absolutely related to depth, and immense. They describe one option with a connection to the surface, but that's not required. It's not really compressed air, more like pumped hydro with an small lower reservoir but a huge height difference.