Discovering existing deposits of the gas that can be economically tapped is key. So, researchers whipped up a guide for hydrogen prospectors.

There's growing optimism that huge amounts of natural hydrogen are stored deep underground. Now, researchers have identified the key geological features that could help us hunt for the clean-burning fuel.

Hydrogen is a leading contender to replace fossil fuels because it only produces water when burned in combustion engines or used to power fuel cells.

We already produce a substantial amount of hydrogen for industrial purposes, such as during the production of fertilizer. But most hydrogen is generated from hydrocarbons, leading to waste gases that add 2.4 percent to global carbon-dioxide emissions.

Efforts are underway to create greener sources of the gas by using renewable electricity to split water into hydrogen and oxygen. But the technology is still far from being commercially viable.

Recently, the discovery of large deposits of underground hydrogen has led to hopes there may be significant natural sources of the clean fuel. Last year, researchers estimated Earth's crust could hold 5.6 trillion metric tons (6.2 trillion tons) of the gas, though they admitted much of this is probably impractical to extract.

Now, another team has identified the key ingredients required to discover large, commercially viable deposits of hydrogen. In a paper in Nature Reviews Earth & Environment, they outline the process likely to produce significant amounts of the gas and what rock structures allow it to collect in places we can reach.

"One successful exploration recipe that is repeatable will unlock a commercially competitive, low-carbon hydrogen source that would significantly contribute to the energy transition," lead author Chris Ballentine, from Oxford University, said in a press release.

The researchers estimate that in the last billion years, Earth has produced enough hydrogen to support humanity's energy needs for at least 170,000 years, though much of this may have been lost to natural processes.

The gas mainly forms during chemical reactions, like when water oxidizes iron-rich rocks and gives off hydrogen as a byproduct or radiation from rocks splits water molecules into hydrogen and oxygen. Interestingly, the researchers rejected a popular theory that hydrogen is supplied to the crust by the fluid mantle layer below.