In context: It's taken Switzerland €2 billion and 14 years, but the country's underground "water battery" is now complete and operational. The project took so long to complete in part because workers had to tunnel through more than 11 miles of the Swiss Alps.

A hydro battery is comprised of two large bodies of water at different heights – in this instance, they are located nearly 2,000 feet below ground between the Emosson and Vieux Emosson dams in Valais.

Excess energy can be used to pump water from the lower basin to the higher pool. When power demand increases, water in the higher pool is allowed to flow back into the lower reservoir. As the water flows, it spins turbines which generate hydroelectric power.

The power plant features six pump turbines that can generate 900 MW of power. The facility was constructed by Nant de Drance and is capable of storing 20 million kWh of electricity, which should help stabilize Switzerland's energy grid. It takes roughly 20 hours to empty out the Vieux Emosson reservoir, we're told.

Renewable energy enthusiasts have been doing a lot of outside-the-box thinking as of late.

Last month, Researchers from the International Institute for Applied Systems Analysis (IIASA) shared plans for a gravity-based system that would use elevators in high-rise buildings to generate and store electricity. Days ago, we learned that a company in Finland has created a battery that uses sand to store electricity as heat.