According to Techspot, the ambitious project took this long to get completed due to the fact that workers had to drill through some 17 kilometers of mountain rocks, among other reasons.
The hydro battery is made of two bodies of water that are located at different heights, around 610 meters underground.
The way the water battery works is that whenever there is excess energy, it can be used to pump the water back in the top-most basin, but when demand is high, it will flow back in the bottom basin, generating extra electric power.
The process of energy generation is very similar to other hydro power plants.
The power plant was constructed by Nant de Drance and has six turbines that can generate some 900 MW of energy.
The facility also has the capacity to store 20 million kWh of energy, which should help to stabilize Switzerland's public energy grid.
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