Trillions of tons of hydrogen gas are likely buried in rocks and reservoirs beneath Earth's surface, but researchers aren't sure where it is yet.

A mountain of hydrogen is lurking beneath Earth's surface — and scientists say that just a fraction of it could break our dependence on fossil fuels for 200 years.

New research suggests the planet holds around 6.2 trillion tons (5.6 trillion metric tons) of hydrogen in rocks and underground reservoirs. That's roughly 26 times the amount of oil known to be left in the ground (1.6 trillion barrels, each weighing approximately 0.15 tons) — but where these hydrogen stocks are located remains unknown.

Most of the hydrogen is likely too deep or too far offshore to be accessed, and some of the reserves are probably too small to extract in a way that makes economical sense, the researchers suspect. However, the results indicate there's more than enough hydrogen to go around, even with those limitations, Geoffrey Ellis, a petroleum geochemist at the U.S. Geological Survey (USGS) and lead author of the new study, told Space.com's sister site, Live Science.

Hydrogen is a source of clean energy that can fuel vehicles, power industrial processes and generate electricity. Just 2% of the hydrogen stocks found in the study, equivalent to 124 billion tons (112 billion metric tons) of gas, "would supply all the hydrogen we need to get to net-zero [carbon] for a couple hundred years," Ellis said.

The energy released by that amount of hydrogen is roughly twice the energy stored in all the known natural gas reserves on Earth, Ellis and his co-author Sarah Gelman, also a USGS geologist, noted in the study. The results were published Friday (Dec. 13) in the journal Science Advances.

To estimate the amount of hydrogen inside Earth, the researchers used a model that accounted for the rate at which the gas is produced underground, the amount likely to be trapped in reservoirs, and the amount lost through various processes, such as leaking out of rocks and into the atmosphere.