With an eye on rapidly rising demand for German electric vehicles, power and mining companies alike are striving to bring to the surface lithium trapped in underground springs of boiling hot water thousands of meters below the Rhine river.
Straddling an area 300 km (186 miles) long and up to 40 km wide, the Upper-Rhine Valley in the Black Forest area of southwestern Germany holds enough lithium for more than 400 million electric cars, geologists have estimated, making it one of the world's biggest deposits.
It could reduce the reliance of the German car industry, also located in southwestern Germany, on imported lithium and early-stage talks are under way with automakers.
But skeptics question the economics and are also troubled by possible local opposition, which can be more vociferous in densely populated Europe than in remote Australia or the deserts of South America that have been the source of lithium supplies to date.
Undeterred, German-Australian start-up Vulcan Energy Resources says it can deliver carbon-neutral lithium, based on extraction using geothermal energy harnessed by up to five power stations it plans to construct.
German utility EnBW already has geothermal power stations and is exploring whether lithium can be a profitable by-product.
"The lithium deposit we're talking about here is gigantic and its properties are ideal for our goal of producing high-quality lithium on a large industrial scale in Germany," Vulcan Energy Resources co-founder Horst Kreuter told Reuters.
The company plans to invest 1.7 billion euros ($2 billion), of which it has so far raised about 75 million euros, to build geothermal power stations and facilities to extract the lithium.
It says it could be extracting 15,000 tons of lithium hydroxide per year at two sites by 2024 and then, in a second phase from 2025 onwards, it targets an output of 40,000 tons per year at up to three additional sites.
Kreuter says he is already in talks with cathode and battery manufacturers, as well as the car industry.
He has powerful backing from Hancock Prospecting, led by Executive Chairman Gina Rinehart, one of Australia's leading investors.