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Ali Kedge discovers the huge potential of geothermal energy for Cardiff and Wales hidden beneath our feet. Back in the 90s, over 200 boreholes were sunk across the capital to monitor groundwater levels as part of the Cardiff Bay Barrage construction. 30 years on and after serving their purpose, rather than lying dormant, they're now being monitored by experts with astounding results. It's led to the largest geothermal data set of its kind in the world and could give the city and wider area a heat network to reduce our bills and help the environment.