Southampton's network, above, has its origins in the 70s oil crisis, when the Government was looking at exploiting alternative sources of heat and electricity.
While national interest in the technology waned once oil prices started falling again, Southampton City Council persisted with its project, which was launched in 1986.
The scheme involves a combined heat and power plant, meaning geothermal energy extracted from 1km beneath the surface is used to generate electricity as well as heat buildings directly.
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