Deep down at the bottom of the ocean lies the mysterious world of the abyss. Here, in the world’s greatest desert, plant and animal life are scarce. And yet, in the midst of boiling, toxic geysers, an environment thought hostile to all life forms, a rich ecosystem flourishes. This miracle is possible thanks to bacteria, micro-organisms crucial to all living beings. How can bacteria survive in such extreme conditions? What lessons can we learn from this incredible profusion of life? To answer these questions, a scientific expedition heads to the depths of the Atlantic to study these creatures of the extreme, including the horse mussel and the vent shrimp, which prosper against all odds thanks to amazing symbiotic relationships with these bacteria. The study of these deep-sea interactions will allow experts to draw parallels with our own relationship to bacteria, since our own bodies host billions of them.