The following is an 20x downsampled global Shuttle Radar Topography Mission (SRTM)  map made with Python and Basemap.

Full resolution can be found on wikimedia.

One can notice the depth of the ocean floor, on a global scale, ranges between -2000 and -6000 meters. In some regions though like the Pacific exceed this range and reach 11 kilometers below the sea surface. One such region is the deepest point on Earth, the Mariana trench as shown below followed by a map for a perspective of its location.

Full resolution: https://commons.wikimedia.org/wiki/File:Mariana_trench.png

Location of the Mariana trench. Wikimedia commons.

Credits: The SRTM dataset used was provided by IFREMER.