Seventy percent of the Earth's surface is covered by ocean, but peel back that layer and a world of features will be seen that are more dramatic than the mountains and valleys of land. However, traditional means of scanning the ocean bottom by using various sonar technologies have mapped a relatively small percentage of the ocean floor. Satellites, however, can provide complete global maps of the ocean bottom, though admittedly with not as high a spatial resolution as ship-based sonar.
This image shows a view of the Northwest Atlantic using data from the ERS-1 and Geosat satellites, analyzed by NOAA's Laboratory for Satellite Altimetry. These satellites send radar pulses from space to the surface of Earth. Slight differences in elevation can be detected on land, as accurately as 1 inch. Mapping the ocean bottom is similarly done - areas where ocean mountains occur make the ocean surface bulge at those points. Likewise, where trenches occur, the ocean surface is depressed. These gravitational anomalies are recorded by the satellites and converted to height measurements, resulting in the map shown here.
Many interesting features can be seen in this image, including the gentle slope of the Continental Shelf off of the East Coast U.S. leading to the drop-off into the abyssal plain; the island chains of Bermuda and the Caribbean; the Mid-Atlantic Ridge, the world's longest mountain range that splits East and West Atlantic; the Puerto Rico Trench, the deepest place in the Atlantic; and even the Cape Verde Islands on the far right near Africa. Additionally, an animation showing the features of the ocean floor can be found here.