Carbon has been stored underground for vastly different lengths of time depending on where it sits. The oldest carbon on Earth has been locked in the mantle and in diamond crystals for over 3.5 billion years. The carbon in fossil fuels is much younger but still ancient: coal deposits date back roughly 300 million years, oil can be 450 million years old or more, and some petroleum traces back over 550 million years. Even soil holds carbon underground for decades, and permafrost stores it for tens of thousands of years.
Carbon in the Deep Mantle
The deepest and oldest underground carbon sits in Earth’s mantle, the thick rocky layer beneath the crust. Some of this carbon has been there since the planet formed about 4.5 billion years ago, delivered by the carbon-bearing dust and organic grains that clumped together to build Earth in the first place. Carbon and nitrogen were part of the raw materials of the early solar system, carried by low-to-moderate volatility organic compounds and carbonaceous grains that existed in interstellar space before the Sun even ignited.
Once locked in the mantle, carbon can stay put for extraordinary stretches. Studies of deep-mantle diamonds, which crystallize under extreme pressure hundreds of kilometers below the surface, show mantle residence times ranging from around 900 million years to well over 1.7 billion years. Carbon that gets pulled back down into the mantle through subduction zones, where one tectonic plate slides beneath another, can remain trapped for hundreds of millions of years before eventually returning to the surface through volcanic activity.
Diamonds: 3.5 Billion Years Old
Natural diamonds are essentially carbon atoms arranged in an extremely rigid crystal lattice, forged deep in the mantle and later carried to the surface by volcanic eruptions. The oldest dated diamonds, from the Diavik and Ekati mines in Canada, formed between 3.5 and 3.3 billion years ago. That’s nearly three-quarters of Earth’s entire history. These diamonds crystallized before oxygen even became a significant part of the atmosphere, which didn’t happen until roughly 2.5 to 2.3 billion years ago. Some of the famous Kimberley diamonds from South Africa are also over 3 billion years old.
A diamond you could hold in your hand is likely the oldest mineral sample available on Earth’s surface, making it a direct physical record of carbon that has been underground for billions of years.
Carbonate Rocks: Hundreds of Millions of Years
A massive amount of Earth’s underground carbon isn’t in fossil fuels at all. It’s locked in carbonate rocks like limestone and dolomite. These rocks form when carbon dissolved in seawater combines with calcium and other minerals, then settles as sediment on the ocean floor. Over millions of years, that sediment compresses into solid rock and gets buried.
Major carbonate formations in places like Pennsylvania date to the Cambrian and Ordovician periods, roughly 540 to 440 million years ago. But carbonate rocks span nearly every geological era. Some of the oldest known sedimentary carbonates are over 3 billion years old. These formations represent the single largest reservoir of carbon in Earth’s crust, storing far more carbon than all fossil fuels combined.
Coal: About 300 Million Years
Most of the world’s major coal deposits formed during the Carboniferous Period, which lasted from about 359 to 299 million years ago. The name itself means “carbon-bearing.” During this era, vast tropical swamps covered much of what is now North America, Europe, and parts of Asia. Dense forests of giant ferns and early trees grew, died, and fell into waterlogged ground where they couldn’t fully decompose. Layer after layer of dead plant material built up, was buried under sediment, and slowly transformed under heat and pressure into coal.
So when you burn coal, you’re releasing carbon that plants pulled from the atmosphere roughly 300 million years ago and that has been compressed underground ever since. Some coal deposits are younger, from the Permian, Cretaceous, or even Tertiary periods, but the bulk of high-quality coal traces to that Carboniferous window.
Oil and Natural Gas: 50 to 570 Million Years
Crude oil and natural gas form from the remains of tiny marine organisms, mostly algae and plankton, that sank to the ocean floor and were buried under layers of sediment. Heat and pressure over millions of years converted that organic material into hydrocarbons.
Most conventional oil reservoirs contain petroleum that formed between about 50 and 200 million years ago, during the Mesozoic and late Paleozoic eras. But some petroleum is far older. In China’s Sichuan Basin, bitumen (a solid form of petroleum) has been dated to between 572 and 559 million years ago, placing it in the late Neoproterozoic era. That makes it older than most complex animal life on Earth. Heavy oil in China’s Tarim Basin dates to 450 to 436 million years ago, corresponding to the Late Ordovician and Early Silurian periods.
These are among the oldest known liquid hydrocarbons, and they show that carbon from ancient marine life has been trapped underground for over half a billion years in some reservoirs.
Permafrost: Up to 40,000 Years or More
Not all underground carbon is ancient in the geological sense. Arctic permafrost, the permanently frozen ground that covers large areas of Siberia, Alaska, and northern Canada, stores enormous quantities of organic carbon from dead plants and animals that froze before they could decompose. Research published in Nature Climate Change found that when permafrost thaws, nearly 90% of the carbon dioxide released comes from organic matter roughly 40,000 years old. That carbon entered the ground during the last ice age and has been frozen in place ever since.
Permafrost holds an estimated 1,500 billion metric tons of organic carbon, about twice as much as the entire atmosphere currently contains. As global temperatures rise and permafrost thaws, this tens-of-thousands-of-years-old carbon re-enters the atmosphere as carbon dioxide and methane.
Soil Carbon: Decades to Centuries
The shallowest underground carbon sits in ordinary soil. Plants absorb carbon dioxide, and when their leaves, roots, and stems decompose, some of that carbon gets incorporated into the soil. In temperate agricultural soils, organic carbon has an average residence time of about 21 years, with grassland soils holding carbon somewhat longer (around 29 years) than cropland soils (around 19 years). Deeper soil layers can store carbon for centuries, but topsoil carbon cycles relatively quickly compared to anything else underground.
This means the underground carbon on Earth exists on a spectrum: from decades in topsoil, to tens of thousands of years in permafrost, to hundreds of millions of years in fossil fuels and carbonate rocks, to billions of years in the deep mantle and diamond crystals. When people talk about fossil fuels releasing “ancient carbon,” they’re referring to carbon that has been out of the atmosphere for 50 million to over 500 million years, now being returned in a matter of decades.