The underground world beneath our feet is a hidden biosphere that plays a crucial role in Earth's systems. Recent research has uncovered the diverse functions of microbes in the deep terrestrial subsurface, revealing how they interact with minerals to obtain energy and carbon. By studying deep mines, laboratories, and boreholes, scientists have gained insights into the extreme environments where these microbial ecosystems thrive. This work sheds light on the interconnectedness of the surface and subsurface biospheres, showing how life exists and thrives in the depths of the Earth.