Usually, when you look at a world map, the continents are shown with mountains, valleys and plains, and the oceans are shown as flat blue surfaces. But that doesn't really give a true picture of the ocean. When you take a closer look past the surface of the water and down to the sea floor, ocean basins are anything but flat.
Ocean floors have just as many mountains, valleys and plains as the continents do.
To focus on the ocean floor, we've blacked out the continents on this map. This map is an ocean basin bathymetry map. It shows the depth from sea surface to sea floor. It's a different way of looking at the earth's surface. You're probably wondering about the features highlighted by the yellow lines in the middle of the ocean basins.