Topography refers to the elevation on land, and bathymetry is the
depth of a depression. Sea level is an average level where topography
and bathymetry both begin at zero for elevation and depth.
The bathymetry of the ocean floors varies, just like land. The
eastern United States coast is shallower than the west coast of the
United States. The deepest trenches are found in the Marianas
Trench in the south Pacific, which is the lowest point on the Earth’s
surface. The Hawaiian Islands are just the tops of very high mountains.
Seamounts are basically mountains that rise from the ocean floor but
never surface. There are vast areas on the bottom of the oceans with no
light and strange animals on an abyssal plain. There are also areas on
the bottom where water of different densities can flow and erode just
like rivers on the surface of the Earth.