A quarter of the UK’s homes sit above abandoned coal mines, long since flooded with water. Now, the mines are being put to a new, zero-carbon use. Coal mines were the beating heart of Britain’s industrial revolution. Their sooty, energy-dense output gave life to new-fangled factories and shipyards, fuelling the nation’s march towards modernity. They helped shape a carbon-intensive economy, one that took little notice of the natural world around it. The mines paved the way for a global dependence on fossil fuels, and in doing so, fired the starting pistol on the climate crisis that today confronts us all. But what if, in a serendipitous circle of history, our extractive past could be repurposed for a greener, cleaner future? What if the vast maze of coal mines beneath our feet, now filled with naturally warm water, could help decarbonise the UK’s — and the world’s — herculean heating needs? That’s the question Adam Black, a renewable energy enthusiast employed by one of Britain’s largest bottling firms, asked himself a decade ago.