The main reason for our lack of direct observations of our ocean floor is technological. The surface of the Earth, the Moon and/or Mars are directly illuminated by light and radio waves. So we can either use passive systems (e.g. our eyes or microscopes) or active systems like radars installed on satellites to look at the echo generated by these surfaces. Satellites can move very fast and cover very large distances very quickly, and so can map the Earth or other planets/celestial objects in just a few weeks or hours, depending of the size of the object.
Liquid water is a much different medium than air or space, and it is much more difficult for electromagnetic waves (light, radio) to transmit through water. That is why, for example, the deep ocean is completely dark. Another consequence of this is that we cannot use our cellphones underwater.
So to “see” through water, we have to use different methods. The most direct and common method uses acoustic (sound) waves to listen to the echo from the ocean floor. But that requires having an asset (e.g. a ship) blasting the sound waves (a very demanding task on the energy side) and listening to the echo. Ships and other marine vehicles are not fast, so they cannot cover vast distances in a short amount of time, unlike satellites.
Scientists have been working to map the ocean floor for many decades. Lamont has been a pioneer in that regard. But there is still a large swath of the ocean floor that remains unexplored.
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