Mines are keeping 350 homes in Gateshead warm once again but now without burning a single lump of coal.
Councillor John McElroy is the local council's environment and transport leader.
He said: "We did it for several reasons. It's great for the environment that we use mines as the natural sort of source of heat, but also it works out economically as well. Great for the customer who's getting good value energy."
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when the mines shut down, the shafts and tunnels tend to flood and that water is warm - anything from 15 - 40C (59-104F) and generally hotter with greater depth as a result of geothermal energy from the Earth's crust.
In Gateshead, they are now piping the water into a heat pump - an industrial version of what's being recommended for homes as a replacement for gas boilers.
Hot water is then circulated to local homes and businesses through a network of pipes.
It isn't zero carbon as it still demands some electricity from the grid, though an on-site solar park boosts the renewable proportion and it is much more climate friendly than just using gas.
