The fact that you can feel the ground moving under your feet is a little unsettling. Not in the way that a garden or forest floor is alive with bugs and roots that you can see and name. The thing that lies much deeper is something else. Something moves so slowly that the word “life” seems like it needs to be rethought.
The deep biosphere is not talked about in the same way that oceanographers or ecologists talk about coral reefs or rainforests. The way they talk makes me think that they are still thinking about what they’ve found because of how measured their words are. And maybe that’s fair. A long time ago, when scientists started taking core samples from holes hundreds of meters below the ocean floor, many of their peers thought the microbes they were finding were just contamination. It took years, sometimes four or more, for journals to publish their work. It turned out that this kind of institutional skepticism was one of the biggest mistakes in modern science.
It is now known that the deep biosphere, which is the huge living zone that goes kilometers below the land and seafloor, is home to between 15 and 23 billion tons of carbon-based life. About 90% of all the bacteria and archaea that are known to exist on Earth live down there, not here in the sun. Every time you sit with that figure, it still feels weird. This means that the world on the surface, with all of its cities, jungles, and oceans full of whales, only makes up a small part of Earth’s total living mass.
How deep life stays alive is what makes it so conceptually strange. It doesn’t get any sun. There isn’t much oxygen around. In the usual sense, there is no organic matter to be found. Instead, microbes get their energy from chemical reactions between rock and water. They do this by eating methane and sulfur compounds or hydrogen that is released by geological processes. Nitrates are what they breathe in place of oxygen. Some of them have metabolisms up to a million times slower than anything on the surface. This means that a single cell might not divide for thousands of years. For these living things, there is no real upper limit on how old they can get.
In the 1920s, geologists and microbiologists at the University of Chicago noticed something strange about water that was taken from oil fields. This was the first real sign of this world. Hydrogen sulfide and bicarbonates are chemicals that are usually made by bacteria. The water came from deep within the earth, which is where most people think life can’t exist. Still, they grew the bacteria. The scientists didn’t know what to think about that. Then, in 1968, a submersible named Alvin sank. A year later, it was found, and the lunches that were still inside were still fresh. Strangely, that observation made people even more sure that the deep sea was biologically dead. Now we can see that it was one of the worst mistakes in the history of biology.
Interest came back in the end, in part because of nuclear waste. The U.S. Department of Energy wanted to know if microbes that live deep underground could break down radioactive material that was buried and get past the containment. That useful but a little awkward question led to research that changed the way scientists think about the living world. In 1987, holes drilled near the Savannah River Site found microorganisms living 500 meters below the surface. The picture kept getting bigger from there.
Still, it’s not clear how far down life can last. Temperatures above 120 degrees Celsius, which is about the same as a pressure cooker on its highest setting, seem to set a limit of some kind. But that limit has already caught researchers off guard more than once. In 2016, a drilling trip into the Nankai Accretionary Prism off the coast of Japan found cells that were still alive at 118 degrees Celsius, which was right on the edge of what scientists thought was the temperature range. Researchers believe that the actual boundary, whichever it is, is likely deeper than what is thought at this time.
When you think about all of this, what you can’t shake is what it means about life as an idea. The organisms that live four miles below the surface weren’t just found recently; they’ve been there, quietly breaking down food, for a very long time, much longer than human civilization. They don’t need the sun. We are not needed. All they’ve done is wait in the dark and do the slow, patient work of living.
