The shape of the ocean floor was not accurately determined until the 1920s. Until the end of the last century mapping had depended on the accounts by sailors of rock formations and deep troughs in the ocean bed.
Recent scientific developments and new instruments and techniques have enabled maps to be drawn with greater accuracy and detail. By transmitting sound waves from ships to the sea bed, and back, it is possible to make a record of the changes in depth.
Mechanical, acoustical, and electronic instruments have pictured the ocean floor not as a vast plain but as series of mountain ranges, valleys, peaks and canyons. Some of the mountains are far higher than most of those on land and the deepest part of the ocean is much farther below sea level than the highest land mountain is above it.