That limited operational window, though, was a conundrum. Fed by the sea and flushed by the tides, the pool was not heated, making it too cold for winter swimming — a major missed opportunity, Ms. Stuart said.

The solution lay beneath her feet.

Image The drilling rig at the Jubilee Pool. Credit... Geothermal Engineering Limited

Now, on a small patch of land adjacent to the pool, a bore hole has been drilled into the rock on which the lido is built. Starting next year, it is expected to provide geothermal energy to help heat the lido’s new wintertime section, allowing locals to swim in any weather.

The project, which will cost about 1.8 million pounds (about $2.2 million) , is being paid for by a combination of grants and various other funding sources, including selling shares to local residents at £20 each. The drilling was carried out by Geothermal Engineering, a local start-up that has big ambitions for geothermal energy.

The Cornubian batholith is a prime location for geothermal projects. Granite contains a small amount of radioactive elements, such as uranium and thorium, which produce heat as they decay. To capture that heat, water is pumped down from the surface through fissures in the batholith. The water absorbs the rock’s heat as it flows, then is brought back to the surface as superheated water, which immediately turns to steam. It can then can be used to power turbines or to provide direct heat.

Cornwall is not the only place in the world with major granite deposits, but it offers an advantage: The rock here extends deep into the earth but is still shallow enough to make accessing it viable.

The first attempts to explore geothermal power in the region took place during the oil crisis of the 1970s; as prices for fossil fuels spiked, so too did interest in alternative energy sources. Initial data was positive, but inconsistent funding stymied further progress.