What was the first major discovery found by using sonar to map the ocean floor that lead to the theory of plate tectonics?

In deep trenches like those found off the coast of South America and Japan, the spreading ocean floor was forced down below the thick continents in regions he called subduction zones. Hess's theory of "seafloor spreading" offered a compelling driving mechanism for Wegener's continental drift, but it needed more proof.

(The first modern breakthrough in sea floor mapping came with the use of underwater sound projectors, called “sonar”, which was first used in World War I. ... This map showed that the canyons off the U.S. East Coast extended into very deep water.)

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