Arizona, United States

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Human activity might have changed the deep subsurface of Earth, a zone located hundreds of metres to several kilometres under the surface. 

According to a study published in Earth's Future, the motion of fluids in the subsurface is much higher under human activity than in natural circulation. However, it is currently unclear what the impacts of these changes might be.

The researchers looked at the extraction of fossil fuels, such as oil and gas, and their replacement underground using salt water. They do it to maintain the pressure in the reservoir. Often, water is already present in these reservoirs, which has remained untouched for millions of years. Adding new water alters the composition and its movement in the subsurface. 

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Another aspect of this process is hydraulic fracturing, also known as fracking, which involves the extraction of oil and gas, not from reservoirs but from deep rock formations. However, breaking the rocks leads to quakes in regions that are not seismically active. It also introduces water and bacteria to the environments underneath. 

Professor Jennifer McIntosh, the senior study author from the University of Arizona, told IFLScience, "We looked at how the rates of fluid production with oil and gas compared to natural background circulation of water and showed how humans have made a big impact on the circulation of fluids in the subsurface."

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Professor Grant Ferguson, the study's lead author, said, "The deep subsurface is out of sight and out of mind for most people, and we thought it was important to provide some context to these proposed activities, especially our environmental impacts."

Many uncertainties surrounding the subsurface raise concerns about potential solutions to the climate crisis. One solution involves storing carbon dioxide underground. However, this method requires burying the carbon dioxide beneath the Earth's surface.

Lithium extraction for batteries also poses a challenge as it requires injecting a large amount of water into the subsurface. Furthermore, geothermal energy as a clean source of electricity could also impact the subsurface. Thus, it is essential to understand what is happening beneath our feet.

"We need to use the deep subsurface as part of the solution for the climate crisis. Yet, we know more about the surface of Mars than we do about water, rocks and life deep beneath our feet," McIntosh said.

(With inputs from agencies)