I saw one of these in Germany recently, never heard of them before: a "pumped-storage hydroelectric power plant". Like a Prius, but slightly larger, heavier and not going anywhere....
This is one of the techniques that can allow an intermittent power source, like wind or solar, to provide dispatchable, base load power. I'm not sure how efficient the process is, but being about to provide energy on demand makes the electricity a lot more valuable.
