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.
One of the ways scientists are attempting to reduce greenhouse gases is to inject these gases into the ground. Specifically, they are testing injecting them, into basalt which is type of igneous rocks usually forming the first (rock) layer (sandwiched between the sedimentary & gabbro layers) in the oceanic crust basalt and in volcanic regions.
The above video features Iceland and its geothermal plants. Iceland is a heaven for geothermal energy as it lies along the Mid-Atlantic Ridge (MAR) (i.e. where Mid-Atlantic ocean floor is spreading apart in opposite directions forming a ridge). Most notably Iceland lies along the V-Shaped Reykjavik ridge (figure below; Google maps) which is part of the Norther MAR.
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).