Aldon (Allie) McIntyre, a white American geologist with a thirst for adventure, and Thadie Mayane, a Black South African mining supervisor with a commanding presence, are exploring the depths of an abandoned mine when the floor collapses, hurling them into an extraordinary realm known as Below. Nestled between the Earth's crust and mantle, this vast world is home to breathtaking landscapes, intelligent species—some friendly, others predatory—and dangers unlike anything they've ever imagined.

Forced to rely on each other for survival, Allie and Thadie must navigate treacherous terrain, fend off alien predators, and face the looming threat of capture by those who see them as little more than slaves. As they search for the legendary passage back to Above, their uneasy alliance will be tested by the perilous environment—and the prejudices and mistrust they each carry.

Will they overcome the trials of Below and find their way back Above? Or will this stunning and dangerous world consume them entirely—if they don't destroy each other first?