By the middle of the twentieth century humanity had charted the surface of the Moon in greater detail than most of the floor of its own oceans, and the reason was simple: light and radio waves, the tools of surveying on land and from orbit, do not travel far through water. Early ocean charts were made by lowering weighted lines from ships, which produced a scattering of depth readings, each one a single point in an enormous unknown. The development of echo sounding, in which the time taken by a sound pulse to return from the bottom indicates depth, transformed the work, since a ship could now record a continuous line.
Even so, the ocean floor remains the least explored large surface on the planet. Ships map it in strips, and the seas are so vast that complete high-resolution coverage will take many decades at the present rate. The irony is that the Earth's most extensive landscape, with mountain chains longer than any on land, is still known to us mainly through inference and sampling, while the surfaces of the Moon and Mars are in some respects better mapped.