Geologists at the UK Coal Authority believe one-quarter of British homes currently sit on a coalfield, stretching across Wales, central Scotland, northern England, and the Midlands. An estimated 2 billion cubic metres (4.4 billion gallons) of warm water occupy the old mine shafts. Researchers suggest that this makes mine water one of the UK's largest underused clean energy sources.
Coal mines were the beating heart of Britain's industrial revolution, giving life to new-fangled factories and shipyards, fuelling the nation's march towards modernity. They helped shape a carbon-intensive economy and paved the way for a global dependence on fossil fuels.
But what if, in a serendipitous circle of history, Britain's 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?
"Mining shaped our urban landscape, creating the towns and cities that we live in today," says Charlotte Adams, the Coal Authority's principle manager for mine energy. "Nine out of 10 of our largest urban centres are above areas of former coal mining activity.
"Mine water is one of our best options to help with the decarbonisation of heating. The resource is readily available all year round at a steady temperature, and there is an abundance to be accessed."
To help meet the country's sweeping carbon-reduction target, the Coal Authority is exploring the feasibility of some 70 mine water heating projects across the UK.
One of the most ambitious water heating projects being developed by the UK Coal Authority is at Seaham, a seaside town in County Durham, home to the old Dawdon colliery. An existing treatment facility pumps up millions of litres of mine water every year for ecological reasons. Mine water often contains toxic compounds, as a result of chemical reactions with the subterranean rock. At the surface, the warmth from this water is now to be used to heat buildings above ground. Once its heat has been absorbed, the water is then returned to the mine workings where it will be warmed up again.
The water is hot enough to heat homes in winter, and cool enough to keep them mild when temperatures rise, with just 25% the carbon emissions of gas.
A few hundred miles away across the North Sea, Heerlen was the crucible of Dutch coal mining before the mass closure of collieries 50 years ago. In 2008, geothermal experts revived the mines, striving to deliver low-impact heating and cooling for hundreds of local homes and businesses.
Today, Mijnwater BV, the scheme's operator, has connected 500 houses and commercial facilities to the town's district heating network.
The system distributes locally generated heat to a nearby community, in a similar way to the one planned at Seaham, reducing the area's carbon emissions from heating by almost two-thirds.
Meanwhile, in northern Spain, there's a similar story unfolding. After years of decline, the area's last remaining pit shut in 2018, cutting deep into a local community reliant on coal for generations. With the advent of mine water heating technology, there's hope for an industrial rebirth.
"Geothermal energy has given a second life to our coal mines," says María Belarmina Díaz Aguado, the Asturias's director of energy. "We're developing an entirely new business model, one related to pumping water and all the technical expertise that involves."
So, there are many former coal regions trying to figure out the best way of tapping the resource. It's currently very expensive, but everyone is optimistic that prices will decrease as this and traditional geothermal technology gain a head of steam.