Life is ubiquitous on our planet, from the surface waters to the deepest oceanic trench, and from the soils to the rocks of Earth's crust. The total biomass of so-called "deep life" on Earth exceeds that of all other plants and animals on the planet's surface. The deepest life on Earth has been found at about 5 km below the terrestrial subsurface and at about 10.5 km below the ocean's surface. Deep life grows in dark and energetically challenging conditions, and has evolved uniquely for millions of years. Thus far, Earth's deep life has remained enigmatic; its exploration will inspire new insights into the origin of life and the conditions for planetary habitability.