Waves in the Southern Ocean are generally fairly large (the red areas in Figure 10.2.210.2.2\PageIndex{2}) because of the strong winds and the lack of landmasses, which provide the winds with a very long fetch, allowing them to blow unimpeded over the ocean for very long distances.
I wonder how the concept of fetch helps explain why the Southern Ocean experiences some of the largest waves on Earth, even without nearby storms?
Large-scale mapping of the ocean floor is also carried out by satellites (originally SEASAT, then GEOSAT, now the Jason satellites) which use radio waves to measure the height of the sea surface (radar altimetry). The sea surface is not flat; gravity causes it to be slightly higher over elevated features on the ocean floor, and slightly lower over trenches and other depressions. Satellites send out radio waves, and similar to an echosounder, can use the returning waves to detect differences in sea surface height down to 3-6 cm (Figure 1.4.51.4.5\PageIndex{5}). These differences in sea surface heights allow us to determine the topography under the surface. Unlike the old lead line technology, where hundreds of soundings were necessary to map a small area, each taking an hour or more to complete, the current satellites can map over 90% of Earth’s ice-free sea surface every 10 days!
I think using satellites for large-scale mapping is a great idea because they can measure tiny changes in sea surface height to reveal underwater features like mountains and trenches. It's faster and covers more area than sonar method, which needs more resources (ships, crew, etc) to do so.