Outside the city of Thetford Mines, Quebec, in a region that once supplied the world with asbestos, workers are drilling underground in search of an unusual and potentially vast new source of clean energy.

A start-up called Vema Hydrogen has drilled two test wells into the bedrock, each 1,000 feet deep, and is starting to inject treated water into the iron-rich rocks below. The goal is to trigger a special type of chemical reaction that could eventually produce large quantities of hydrogen, a clean-burning fuel that may one day play a vital role in tackling climate change.

“The potential is massive,” said Pierre Levin, the chief executive of Vema, as he watched a drilling crew at work on a bright, bitterly cold spring day. “You can find rocks like this all over the world, enough to produce billions of tons of hydrogen.”

The dream of clean hydrogen has tantalized energy experts for decades. When burned, hydrogen emits only water vapor. It could theoretically be used instead of fossil fuels in ships, airplanes, steel mills or chemical plants, all industries where it’s difficult to find viable alternatives for oil, gas and coal and cut planet-warming emissions.

The problem has always been making that hydrogen.

Most of the hydrogen the world uses today — mainly for fertilizer and refining — is produced using natural gas in a process that creates lots of emissions. In recent years, the United States and other countries have invested billions of dollars trying to make “green” hydrogen with wind and solar power, but it has proved difficult and expensive.

Now a growing number of companies think a better answer could lie underground. Dozens of start-ups are trying to find large reservoirs of natural hydrogen thought to exist below the surface. Others, like Vema, are trying to stimulate the processes that generate that hydrogen, without any emissions. It’s a field often referred to as “geologic hydrogen.”