My question is…why/how is it visible? If you zoom out enough in Google Maps and look at either the Atlantic or Pacific oceans, you can clearly see the undersea ridges, mountain ranges, and plate junctions.
My (possibly ignorant) assumption is that the oceans are nowhere near transparent enough when viewed from space to reveal those details, so I’m guessing it’s a technical property of certain kinds of satellite photography, like the use of non-visual spectra (infrared?, UV?).
No, it’s not real imagery. It’s just a relief map, made using traditional methods (soudings, radar etc). The oceans are just filled in to a uniform blue (away from the immediate coastline - you can see where the joins are) with the relief superimposed.
For example, see here. The real satellite imagery on the islands is clear, but the ocean is just a blue fill “draped” over a fairly low-res relief map - you can see the large pixellation of the depth data.
As for where Google gets that data from, if you zoom to an area of empty ocean, you get “(c) 2008 TerraMetrics, Nasa”. TerraMetrics appears to be here.