Two-thirds of Earth's surface lies underwater, several kilometers below sea level, and only in the last fifty years have scientists mapped it in any detail. This film follows oceanographers studying the mid-ocean ridge, a mountain range that circles the entire globe along the sea floor and is the site where new crust is continuously formed as tectonic plates pull apart. Researchers explain the geological processes building this landscape and describe the organisms that have adapted to live in total darkness and crushing pressure far from any coastline. The film also shows the practical difficulties of the work itself: the equipment and dives required to reach terrain that stays largely inaccessible even now, and the hazards that come with working thousands of meters down. Rather than treating the deep ocean as scenery, the film treats it as an active, still-forming part of the planet's surface, one that happens to be harder to reach than the surface of some other worlds.