Green and blue hydrogen have received huge attention as potential low-carbon fuels in recent years, but they also have significant downsides. Both are expensive and faced challenges and delays in their rollout. And while their use is slowly growing, together they still only make up around 1% of global hydrogen production. Some researchers have also raised doubts over how low-carbon blue hydrogen really is due to associated leaks of methane – a potent greenhouse gas which is 80 times more powerful than carbon dioxide (CO2) over a 20-year time span. Meanwhile, in recent years scientists have found that naturally occurring hydrogen is actually much more widespread than previously thought, leading some to believe it could be tapped as a cheap and carbon-free fuel. This "geologic" hydrogen, also called natural or white hydrogen, is produced naturally when underground water encounters iron-rich rocks in a process known as serpentinisation. Because hydrogen is so light, it usually seeps through porous rocks and cracks, eventually rising to the atmosphere. That's if it isn't first consumed in underground reactions or eaten by subterranean microbes. But in some geological settings, hydrogen can become trapped under rocks with low permeability, such as salt or shale rocks, which create a seal under which the gas can accumulate. It's these hydrogen accumulations in the Earth's subsurface that prospectors hope may be viable for commercial exploitation. According to a 2024 study from the US Geological Survey (USGS), there could be anywhere between one billion and 10 trillion tonnes of hydrogen in the subsurface, with a best guess of around 5.6 trillion tonnes trapped in geological formations.