There's Life Inside Earth's Crust

Summary

Credits Karen G. Lloyd is the Wrigley Chair in Environmental Studies and professor of Earth science at the University of Southern California. This essay is adapted from her forthcoming book “Intraterrestrials: Discovering the Strangest Life on Earth” (Princeton University Press, 2025). 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 sedim...