Deep beneath the ocean exists an incredibly hostile world - an icy world plunged in eternal darkness where the pressure is phenomenal. This world is the abyssal realm - the largest habitat on our planet. Yet some marine mammals that breathe air and nurse their young, like us, spend a large part of their days in this shadowy world. Scientists are now discovering that the depths reached by these ocean dwelling giants are far greater than they had ever imagined. Using tags attached to the animals, cameras in the deep ocean trenches and 3D scanning images, this film follows and illustrates a modern scientific revolution. Constructed as a dramatic progression towards the deep sea trenches - from the shallowest to the deepest - it shows animals that are extremely rare and highlights the remarkable physiological adaptations that allow them to dive so deep.