Parts of Sweden use District Heating (heat for everyone comes from the same source), and one location is filling caves with hot water to create a giant network of thermal batteries.

Unused energy in the summer is used to heat the boilers, which can be used to heat the city in the winter.

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Somewhere below the Swedish city of Västerås, there's a big man-made cave. During the Cold War, it was used to store oil. But the local energy company decided to clean it, pump it full of water and heat it up. Here's why they did that.