A biologist’s firsthand account of the hunt for life beneath earth’s surface—and how new discoveries are challenging our most basic assumptions about the nature of life on EarthLife thrives in the deepest, darkest recesses of Earth’s crust—from methane seeps in the ocean floor to the highest reaches of Arctic permafrost—and it is unlike anything seen on the surface. Intraterrestrials shares what scientists are learning about these strange types of microbial life—and how research expeditions to some of the most extreme locales on the planet are broadening our understanding of what life is and how its earliest forms may have evolved. Drawing on her experiences and those of her fellow scientists working in challenging and often dangerous conditions, Karen Lloyd takes readers on an adventure from the bottom of the ocean through the jungles of Central America to the high-altitude volcanoes of the Andes. Only discovered in recent decades, “intraterrestrials”—subsurface beings that are truly alien—are demonstrating how life can exist in boiling water, pure acid, and bleach. They enable us to peer back to the very dawn of life on Earth, disclosing deep branches on the tree of life that push the limits of what we thought possible. Some can “breathe” rocks or even electrons. Others may live for hundreds of thousands of years or longer. All of them are living in ways that are totally foreign to us surface dwellers. Blending captivating storytelling with the latest science, Intraterrestrials reveals what microbes in Earth’s deep subsurface biosphere can tell us about the prospects for finding life on other planets—and the future of life on our own.