One of the mysteries of the ocean is the long, deep gorges that wind across the continental shelves like submerged river valleys. Oceanographers thought at first that they were really valleys cut by ancient rivers when sea level was lower, and flooded by the rising water when ice-age glaciers melted. This theory went out of fashion when improved sounding methods showed that some of the streamlike channels lead down to the ocean floor itself three miles below the surface. The level of the ocean could never have fallen as low as that.
A more recent theory has it that the gorges were...
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