The way to spot a cold methane seep on the ocean floor is to look for the life that gathers around it, like antelopes at a savanna watering hole: clams, mussels, crabs, shrimp, fish, sea anemones and creepy, otherworldly worms. These seeps, exposed by movements of tectonic plates or other geological processes, allow ancient, deeply buried methane to burble through the Earth’s crust and into the water column, where it becomes a kind of manna from heaven — a highly energetic food in what is otherwise a barren desert. Single-celled microbes eat the gas; the crustaceans, worms and other creatures in turn eat the microbes. For microbiologists like myself, this motley crew of creatures is a precious sight, but not because I’m interested in studying them — it’s the tiny microbes I make these half-mile descents for.

Ever since I became a microbiologist, a series of questions has gnawed at me: Are there life-­forms hiding inside the Earth? And if there are, how do they survive? Would their nature be so strange that they change our conception of life itself? 

The major categories of visible life on Earth have been pretty much settled for centuries. But it wasn’t until the 1980s that scientists found “intraterrestrials” — microscopic organisms living in what the biogeochemist David Valentine calls a “microbial purgatory deep below the Earth’s surface.” Soon followed by other revelations of life inside Earth’s crust, these discoveries revealed that we had been missing major branches on the tree of life. Indeed, these microbes proved that our assumptions about the boundaries of life were wrong — and wildly so.

As it turns out, much of Earth’s habitable space lies deep under thousands of feet of sediments and rock. You might think that any sad little trickle of life in that deep, dark underworld might like to clamber back up to the surface, desperate for life­giving sunlight. But anyone who’s traveled to a methane seep at the bottom of the sea knows the opposite is true: life flourishes in the dark, and it does so on terms antithetical to those that we who inhabit the thin green layer at the Earth’s surface know.

Intraterrestrials survive without sun and oxygen; instead, via thermodynamics (the art of moving energy around), they are able to respire most elements of the periodic table. Radioactive uranium doesn’t really have a “life support” ring to it, yet that’s just what it is for some intraterrestrials. Arsenic is poisonous to humans and many other creatures, but intraterrestrials respire it too, essentially cleaning up our toxic pollution. Gold is not very chemically reactive — that’s why humans use it in currency and jewelry. Yet there are microbes that consume it. 

We know they are alive because we can see the effects of their respiration on the chemicals around them, and we can see that they are intact and not degraded. We know they must be refreshing their cellular biomass. But they are doing it immensely slowly — gradually replacing their parts, lipid by lipid, nucleotide by nucleotide. It takes roughly half a century for them to replace all their molecules.

These single-celled organisms can live for hundreds of thousands, perhaps millions, of years. (By contrast, the oldest multicellular organisms are trees, some of which can live to be thousands of years old, although none of the cells that compose them live for that long.) Theoretically, therefore, when I scoop deep sediment samples, I could be touching microbes that have been living and breathing continuously since well before humans were even a species. 

By clinging to the bitter edge of what scientists ever would have guessed is the necessary amount of energy to support life, the intraterrestrials have earned a different relationship with time than we have. Indeed, many endure in environments where they don’t get enough energy to produce new cells, instead surviving on 0.00001­% of the power that supports all other known types of