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If you had to nominate the slowest, longest-living organisms on Earth, what would you picture? Among the vertebrates, some people might think of tortoises, whales or perhaps more obscure creatures like the Greenland shark, which can live for centuries. Others might imagine coral colonies, or perhaps an ancient tree: there are oaks in England that could be more than 1,000 years old, whereas in California, a few Bristlecone pines have been around for millennia, dating to around the formation of ancient Egypt.

But how about bacteria? Microbes, at the outset, may seem unsuitable candidates for the title of longest-living organism, since we’re so used to experiencing how they grow (and die) so quickly. If I wake up with a tickle in my throat, I get a feeling of dread because I know that, by the evening, I’m going to have a full-blown case of strep throat – the bacterial cells dividing like wildfire in my body. Some bacteria, like E coli, can double every 20 minutes. They can be killed off just as quickly, when faced with antibiotics or disinfectant.

However, E coli and other fast-replicating microbes don’t live in subsurface environments, where the conditions are ripe for a far more languid pace. In recent years, my fellow biologists and I have assembled evidence suggesting that the microbial world deep beneath the ground may be far slower than we think – perhaps remaining metabolically active for millions of years. I call these organisms aeonophiles – and by living as long as they do, they are rewriting the rules of biology itself. What are they doing down there? It turns out they might be waiting – waiting to return to the surface. But unlike cicadas or hibernating bears, these living things are holding on for events that might take centuries, millennia or even geological eras to arrive.

The steps that led to our discovery of this strange life can be traced back to advances in DNA technology in the 1980s. For the first time, biologists could sequence the DNA from microbes directly, in any environment, without first growing these microbes in a laboratory. In 1998, Philip Hugenholtz, Norman Pace and colleagues at the University of California, Berkeley used this new technology to discover 12 deep branches on the tree of life in a Yellowstone National Park hot spring. The next year, Costantino Vetriani and Anna-Louise Reysenbach at Rutgers University in New Jersey and colleagues discovered even more new groups in deep-sea mud. None of these organisms had parallels in the known world of microbiology; they were entirely new to science. This new DNA-sequencing technology took off like wildfire, and scientists around the world, including myself as a young researcher, started discovering new types of life all over the place.

What we’ve discovered since has changed our conception of what life is like on Earth. Before these discoveries, it was unknown whether life can exist inside Earth’s crust. We now know that there is life under our feet, way under our feet. These subsurface-dwelling single-celled organisms are collectively called intraterrestrials, due to their parallels with the mystery and novelty of extraterrestrials. But, unlike space aliens, we know for certain that intraterrestrials exist.

The author and research team drilling in Svalbard, northern Norway. Photo by Jon Leithe

Intraterrestrials comprise a vast still-mysterious ecosystem in Earth’s crust containing as many (or more) living microbial cells than are on Earth’s surface. We know this from scientists such as myself going out on scientific drilling ships that sample deep marine sediments or drilling deep into continental crust, laboriously counting the number of cells we find there, and extrapolating out to the rest of the world. The deepest we’ve found intraterrestrials thus far is about 5 km down. That’s deep enough for these intraterrestrials to never see the light of day, nor do they receive much food input from the surface world. Their world is mostly composed of tiny spaces between sediment grains or miniscule fractures in rocks. Rocks seem solid to us, but to very tiny life, rocks appear porous, with lots of places to live. From the few growing cultures that we have of these organisms, we know that many of them are tiny, and some have long appendages, such as the Asgard archaea and the Altiarchaeales, which may help them to hang on to their rock or sediment housing.

The intraterrestrial Lokiarchaeum ossiferum (‘skeleton-carrying’) is a member of the Asgard phylum, so named after Norse mythology because some of the first examples were........

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