What is ocean floor mapping?
The ocean floor is now mapped from space. When this was done underwater mountain ranges were discovered (presumed before and point depths found by sounding from ships but extent never fully known until space age). These ranges are active and are due to larva build up from the Earth's interior i.e. the surface is split at these ranges and the split is a plate boundary. It was also from the larva deposits that had been pushed further and further away from the original activity that it was proved from the magnetism of the deposits that the Earth's magnetic field reverses at long regular intervals.
When they use a system to map the ocean floor.