A shoreline seems to mark the point where a land mass ends and the ocean floor begins. But that isn’t exactly so.
Every land mass includes a shelf of land that extends out into the ocean, sloping downward. This shelf of submerged land is called the continental shelf.
Along the East Coast of the United States, the continental shelf extends about 200 miles into the Atlantic Ocean. It slopes downward until it reaches a depth of about 1,200 feet. Then the shelf ends, and a steep incline leads downward to the bottom of the ocean. Beyond the edge of the continental shelf, the depth of the ocean quickly increases to about 12,000 feet.
If the level of the oceans were lower, much or even all of the continental shelf might be dry land, then the ocean wouldn’t begin until a point 200 miles off the present coast of America!