It’s possible to pump heat – or cold – into large areas of these “sponges” and then take the hot or cold out again via a fluid when required, to warm or cool down dwellings. Such a system could be even more efficient than hot water reservoirs in caverns, Prof Jackson says.

Despite numerous aquifer thermal energy storage installations in Europe – there are thousands in the Netherlands, for instance – it remains rare in the UK.

“That’s a technology which is not that prevalent [here],” says Prof Jackson. “We have 11 installations operational at the moment.”

One example is the luxury Chelsea Barracks complex in London, which is being redeveloped into a residential and commercial area. Between 2015 and 2018, an aquifer thermal energy storage system was installed by a Belgian company, AGT.

Aquifers suitable for this sort of thing are plentiful around the UK, however, and often conveniently located right under cities, notes Prof Jackson.

And yet, the UK is arguably falling behind on making use of underground thermal energy resources.

“The main barriers to deployment in the UK are not around technical suitability,” says Prof Jackson. “They’re really around awareness of developers that this is a solution.”