What percentage of the ocean floor has been mapped in detail?

How to get close to this number

Reason from the method. High-resolution seabed mapping is done by ships towing sonar at survey speed, covering a strip a few kilometres wide, and the ocean is hundreds of millions of square kilometres. At that rate, mapping the whole floor takes decades of continuous fleet time — so the completed fraction after a few decades of effort has to be a minority.

The Mars comparison in the explanation is the one that lands, and the reason is physics rather than priorities. Orbiting cameras and radar can map a planet's surface from above in months, but water blocks every wavelength that would do the same job here. The ocean is hard to map because you cannot look at it from a distance.

That is why satellite-derived seafloor maps exist but are low resolution: they infer depth from tiny variations in sea-surface height caused by underwater mass. It works globally and coarsely, which is why most of the ocean floor is known in outline and unknown in detail — two very different states that get conflated in the phrase "unexplored".