Naturally occurring "white hydrogen" lies in vast reservoirs beneath our feet – now the gold rush of the clean energy era is beginning. Investors had lost faith in Edwin Drake's obsessive hunt for oil when the American entrepreneur finally struck black gold in an underground reservoir in Titusville, Pennsylvania, in 1859. The discovery spurred an exploration frenzy that launched the modern oil age. Now, a new generation of wildcatters are racing to replicate that Titusville moment, hoping to bring about the dawn of a major new energy resource. However, it's not fossil fuels they are looking for, but a commercially viable source of natural – and low-carbon – hydrogen. Hydrogen, the smallest, simplest and lightest molecule on Earth, is currently used mainly for refining and chemical industries, such as producing ammonia for fertilisers. The vast majority of this hydrogen is made from polluting methane gas or coal gasification. But there are already other, lower-carbon ways to produce hydrogen. And hydrogen's ability to store three times more energy than oil, while only producing water when burnt, has made some view it as an attractive clean fuel option, especially for industries which are hard to decarbonise by electrification, such as aviation, shipping or steel production. "Green" hydrogen, for example, is a cleaner alternative made by splitting water between hydrogen and oxygen molecules in a process powered by renewable energy. "Blue" hydrogen, made from fossil fuels using carbon capture and storage to reduce the emissions, is another alternative.