In the rocky soil of Lorraine, a former coal mining region near the French-German border, scientists guided a small probe one recent day down a borehole half a mile into the earth’s crust.

Frothing in the water table below was an exciting find: champagne-size bubbles that signaled a potentially mammoth cache of so-called white hydrogen, one of the cleanest-burning fuels in nature.

“Hydrogen is magical — when you burn it you release water, so there are no carbon emissions to warm the planet,” said one of the scientists, Jacques Pironon, a senior researcher and professor at the University of Lorraine. “We think we’ve uncovered one of the largest deposits of natural hydrogen anywhere in the world.”

The discovery by Mr. Pironon and another scientist, Philippe de Donato, both members of France’s respected National Center for Scientific Research, caused a sensation in France, where the government has vowed to become a European leader in clean hydrogen.

There are still many questions about the find, including exactly how big it is and how best to extract the gas. But it has added to a trail of clues elsewhere in the world that a holy grail of clean energy may be lying in the earth for the taking.

Governments and companies worldwide have been betting on hydrogen as a cornerstone in the fight against climate change. A multibillion-dollar industry, backed by billions more in subsidies and private investments, has sprung up to support the manufacturing of hydrogen, which in theory could substitute for fossil fuels to power factories, trucks, ships and planes, potentially removing around half of all planet-warming emissions.

But making commercial hydrogen involves splitting water into hydrogen and oxygen, an endeavor that requires energy. If fossil fuels are used, the process results in greenhouse gas emissions, and the result is called gray hydrogen. Tapping renewable electricity from wind turbines and solar panels to produce what’s called green hydrogen is cleaner but more expensive.

Natural hydrogen, also called white hydrogen because of its purity, could be a game changer, scientists say, because it is a potential source of clean energy continuously generated by the earth. Hydrogen reservoirs form when heated water meets iron-rich rocks. According to the U.S. Geological Survey, just a small fraction of these deposits could provide enough clean energy for hundreds of years.

“If they do verify this discovery, then it is very significant and would have a big impact on society,” Geoffrey Ellis, a geochemist at the U.S. Geological Survey and a global expert on hydrogen, said of the French finding. “There are many other places around the world where similar finds could also be made, and people are looking at it because it really could be impactful.”

In Lorraine, the scientists said their tests suggested that 46 million to 260 million metric tons of natural hydrogen could be lurking beneath the coal mines, which were abandoned in the 1970s when France shifted to nuclear power. By comparison, around 70 million metric tons of hydrogen is produced commercially worldwide each year.

Natural hydrogen reserves have been detected recently in parts of the United States, Australia, Africa, Russia and elsewhere in Europe, too. It’s not unusual to find hydrogen when drilling for gas or oil, but in the past companies ignored such discoveries because of low demand.

Researchers didn’t give white hydrogen much credence until a chance discovery in Bourakébougou, a small village in Mali, in 1987 when a worker accidentally set fire to a water well by lighting a cigarette over it. The well was found to contain natural hydrogen, and it is now used to power shops and homes after a local entrepreneur hired a petroleum company to tap the gas.

“People had not been looking for natural hydrogen for years and years because everyone was focused on drilling for oil and gas,” said Julien Moulin, president of Française De l’Énergie, a clean energy company that is working with Mr. Pironon and Mr. de Donato to test and develop white hydrogen projects. “But it feels like we’re at the beginning of a new dynamic,” he said.

Française De l’Énergie’s primary business has been capturing methane gas from coal seams and converting it to clean power for industries in the region. With the discovery of hydrogen, the company will intensify efforts to explore and extract it, Mr. Moulin said.

“You’ve got the cake — now the question is how do you eat it?” he said. “You need to create the tools to develop this resource, and that will be the work of the next several years.”

The efforts in Lorraine reflect broader excitement rippling through the clean fuel industry about natural hydrogen. The growing understanding that earth is its own hydrogen factory has set off a mini-gold rush among researchers and start-up energy companies eager to make a find.

In Australia, Gold Hydrogen, an independent energy company, is digging for natural hydrogen near Adelaide after unearthing historical papers from two oil wells drilled in the 1930s that showed vast amounts of high-purity hydrogen in the area. Bill Gates is among the investors in the United States who have provided funding to Koloma, a Colorado company probing for hydrogen in a huge geological rift in the Midwest. In Europe, small energy companies from Spain, Switzerland, the Nordic countries and beyond are all scouring the earth’s crust.

Whether white hydrogen lives up to the hype remains to be seen. So far, the finds range from potentially huge ones that may take years to verify, like the one in Lorraine, to small or extremely deep accumulations that may not be economically viable to go after, Mr. Ellis said. Questions linger about whether it is an unlimited source of clean fuel. Big oil companies, like TotalEnergies of France, have not jumped in to invest and appear to be waiting on the sidelines to see how things develop.

Then there is the cost. Although the United States and Europe have set aside billions to subsidize the development of green hydrogen using renewable power, none of that money goes to encourage white hydrogen production.

And producers of white hydrogen must keep an eye on the final price of their gas. Although green hydrogen costs about $5 per kilogram to produce — more than twice as much as gray hydrogen — the U.S. Energy Department is sponsoring a program to get green hydrogen’s price to $1 per kilogram within a decade.

In Spain, a start-up called Helios Aragón is developing a natural hydrogen production project in the Pyrenees that it claims will be able to match or beat that price.

“The No. 1 question is what the cost will be,” said Marco Alverà, chief executive of Tree Energy Solutions, or TES, a company that plans to produce and import clean hydrogen to Europe. For natural hydrogen to be competitive, “it depends on many factors, including the pressure the gas is under, the temperature, the type of rock you drill through,” he said.

In the meantime, Europe is building a large network of pipelines that could deliver manufactured hydrogen to factories and fuel sites. The hope is that white hydrogen could one day flow through them.

If all goes according to plan in Lorraine, new drilling will start next year with an advanced probe that will take gas samples from as far as 1.8 miles below ground — deeper than the Golden Gate Bridge is long — to test the magnitude of the hydrogen trove, with the aim of extracting natural hydrogen by 2027 or 2028.

Mr. Pironon and Mr. de Donato have high hopes. When they started searching for methane gas left by the coal mines, they instead discovered hydrogen the deeper they went. A half-mile down, they found higher concentrations of hydrogen than had been reported anywhere else in the world, Mr. de Donato said.

“We might have a real hydrogen factory hidden under our feet,” he said. “It’s a cause for real excitement.”