How Did Ancient Sunlight End Up Buried Underground as Coal and Oil?

Coal, oil, and natural gas are fossil fuels—energy sources composed of hydrocarbons. But what they really contain is something more surprising: transformed solar energy, stored by ancient organisms hundreds of millions of years ago.

Here's how sunlight became buried underground.

During the Carboniferous Period, roughly 359 to 299 million years ago, massive fern-like plants flourished across the Earth. Thanks to a structural material called lignin, these plants could grow extraordinarily tall and absorb large amounts of solar energy. But there was a critical twist: bacteria capable of decomposing lignin hadn't evolved yet. When these plants fell and were buried, they didn't decay. Instead, they accumulated layer by layer, preserving carbon and solar energy within them.

The accumulated plants first formed multiple layers of peat—a dense, carbon-rich material made of partially decomposed organic matter. Over millions of years, these layers were subjected to intense heat and pressure, and peat transformed sequentially into lignite, sub-bituminous coal, bituminous coal, anthracite, and finally meta-anthracite. That's how plant-stored solar energy became coal.

Oil has a different origin story. There are several theories about how it formed—broadly divided into abiogenic and biogenic theories—but the prevailing view is that oil formed primarily from phytoplankton—microscopic marine organisms—that accumulated on the seafloor over long periods. During the Mesozoic Era, intense volcanic activity raised atmospheric carbon dioxide levels, which in turn warmed the oceans. This allowed phytoplankton remains to be deposited in oxygen-deprived conditions without decaying.

Under heat and pressure, these remains formed an organic substance called kerogen. Continued exposure to extreme conditions converted kerogen into crude oil and natural gas through pyrolysis. About 70% of the crude oil we use today was formed during this era.

But here's something that might seem strange. As of 2017, 48% of proven oil reserves are located in the Middle East—mostly on land, not under the sea. If oil forms in marine environments, how did it end up in a desert?

The answer is that the Middle East wasn't always a desert. Around 100 million years ago, the region was covered by an ancient body of water called the Tethys Sea. When the African and Eurasian plates collided, the seafloor was pushed upward, turning that marine environment into dry land—with all its oil still intact beneath the surface.

Today, there's growing pressure to reduce our dependence on fossil fuels. Burning hydrocarbons releases carbon dioxide, which accelerates global warming and threatens ecosystems worldwide. Oil spills contaminate soil and groundwater. Plastics derived from fossil fuels are breaking down into microplastics that now cycle through wildlife and into the human food chain.

At the same time, fossil fuels remain deeply embedded in modern life—powering transportation, producing fertilizers, and serving as raw materials for plastics and asphalt. Finding a moment in daily life untouched by fossil fuels is nearly impossible.

Carbon cycles globally between the biosphere and the lithosphere. The use of fossil fuels is the first step in balancing environmental protection and development—and on a global scale, it's a decision that calls for careful thought and a search for alternatives.

Disclosure: I'm participating in a paid supporter activity for SOAK, a science content platform, and this post was written as part of that activity.