The nature of the seafloor depends upon how you define a sea. Seas occupy the interior of continents, as is the case with the Black Sea, the margins of continents, as do the North Sea and the Sargasso Sea, and the true ocean basins. Interior seas, which were much more common at times in the geological past, reside on the continental crust, tend to be shallow, and are usually somewhat restricted in their exchange with the open ocean. Interior seas are a product of high sea level -- when sea level is high, interior seas are common. As a result, the floor of an interior sea looks little different than the surface of a continent.

Marginal seas vary in character with sea level as well. During periods in the geological past when sea level was very low, marginal seas may have resided on the outer edge of the...