The novel begins with preparations for the dive, off the coast of Africa. Prof. Maracot claims to have located the deepest trench in the Atlantic and is vehement that he shall go down in the specially prepared submersible (actually a bathysphere) along with Headley and Scanlan. On reaching the edge of the trench, a description of the undersea world is presented. The team comes face to face with a giant crustacean who cuts off their line and hurls them down into the trench. Down in the trench, the team is rescued by the Atlanteans who are the last survivors of the land that was Atlantis.