Over a million years later, as humanity struggles to evolve past fossil fuels in time to avoid climate collapse, that simple idea is making a modern comeback – and hot rocks are hotter than ever, literally and figuratively.

"(The rocks) in the box right now are about 1,600 degrees Celsius," Andrew Ponec said, standing next to a thermal battery the size of a small building. That is nearly 3,000 degrees Fahrenheit, "Hotter than the melting point of steel," he explained.

But what makes his box of white-hot rocks so significant is they were not heated by burning tons of coal or gas, but by catching sunlight with the thousands of photovoltaic solar panels that surround his prototype west of Fresno.