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).
Ibrahim El Merehbi 