What did Harry Hess do with the sea floors spreading?

Answer 1

The idea that the continents might have moved, or "drifted" over time can be traced back to the sixteenth century, when European cartographers compiled world maps based on the seagoing expeditions of that time. This idea was transformed into the theory of "continental drift" by German meteorologist Alfred Wegener in 1912, when he published a treatise with several lines of supporting evidence that went beyond simply matching the continents like puzzle pieces. These lines of evidence included, for example, matching geological formations and paleontological distributions from South America and Africa. Wegener's critics correctly pointed out, however, that the continents could not simply "plow" though the ocean floor as Wegener had vaguely theorized. It was Hess who determined how oceanic mountain ranges, called mid-ocean ridges, are fundamental to the tectonic movement that results in the drift of continents.

Basically, he used echolocation to map the sea floor and proved that continental drift does occur.