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.
The Mariana backarc, the same location of the deepest point on Earth, is where the thermal vents occur due to the spreading of two plates. In other words, the spreading apart of the sea floor results in a trench like the Mariana trench. At such backarcs, and the similar but not the same Mid-Ocean Ridges, “black smokers” form.
The following video is a fly-thru movie at the Mariana backarc in search for HydroThermal vents (second video).