A new study has uncovered evidence that microbial life deep within Earth has survived and thrived through hundreds of millions of years of dramatic geological changes, including mountain formation and erosion. Researchers analyzed samples from a borehole nearly 2.3 kilometers deep in central Sweden, revealing that methane-producing microbes have been active in connection with major geological events that shaped Scandinavia. These findings were published in the journal Communications Earth & Environment. The deep underground, far beneath Earth's surface, is one of the least explored environments on the planet, yet it is home to unique microbial communities that carry out ancient metabolic processes. One such process is methanogenesis, where certain microbes produce methane gas. An international team of scientists, led by researchers from Linnaeus University, has pieced together a rare long-term history of life in this hidden world. The team examined calcite minerals found in fractures within the COSC-2 drill core, which was extracted from central Sweden. This borehole reaches over 2,300 meters into the bedrock and passes through various layers of rock formed at different times in Earth's history. The researchers found unusually high carbon isotope values in the calcite, a sign of microbial methanogenesis. These isotope signatures suggest that microbes used lighter forms of carbon, leaving behind a reservoir rich in heavier isotopes. Using a precise dating technique called U–Pb dating, the scientists determined that methane production occurred in multiple distinct episodes, ranging from about 378 million years ago to as recently as 1.8 million years ago. This shows that life in the deep subsurface is not static but changes in response to geological processes over vast timescales, turning on and off as environmental conditions shift. The borehole cuts through rocks that were once part of the Caledonian mountain range, an ancient mountain belt that once stretched across Scandinavia but has since eroded over hundreds of millions of years. Despite these changes, microbial life continued to persist. The study highlights the remarkable resilience of these microorganisms, which have survived tectonic shifts, burial, uplift, and temperature changes. They have repeatedly colonized deep environments and reactivated when conditions became favorable. This discovery offers a new perspective on how life might exist in subsurface environments on other planets and informs where scientists might search for extraterrestrial life. It also helps us understand how these deep ecosystems may respond to future changes, such as those caused by human activity and climate change.