Karen G. Lloyd's new book, 'Intraterrestrials,' highlights the extraordinary deep biosphere within Earth's crust as a new frontier for scientific exploration. The book argues that studying the microbial life thriving in these subsurface environments offers insights akin to exploring an alien world. This 'intraterrestrial' realm presents unique opportunities to understand the limits of life, the origins of life on Earth, and potentially even the existence of life elsewhere in the universe. Lloyd emphasizes the potential for groundbreaking discoveries within our own planet, urging further research into this largely unexplored ecosystem.