A scale superimposed on the image of the cross-section of the ocean shows that the average depth of the ocean is 3.8 km. It also shows point A (surface level): Snorkeling allows great freedom of movement. However, a diver must constantly return to the surface to breathe. The ocean structure includes:

• Continental shelf: Continental shelves are gradually sloping zones that extend outward from each of the major land masses.
• Continental slope: This steep incline extends downward from the edge of a continental shelf.
• Seamount: Seamounts are submarine mountains whose peaks do not break the ocean’s surface.
• Abyssal plain: Thick layers of sediment, formed by dust and the remains of microscopic organisms, cover these broad, flat expanses of sea floor.
• Guyot: Some seamounts are guyots—extinct volcanoes with flat tops. Guyots were once islands, but are now below water because the entire ocean floor sinks as it gets older.