Sorry about the weird phrasing in the header. I phrase this in an old OP as “how low can a worm go.” On earth, somewhere between the molten iron core and sea level.
Halicephalobus mephisto is a roundworm found 2.2miles below the surface in a goldmine in South Africa. I’m not sure what the elevatation of the mine is.
How about which animal lives the deepest below sea level on land?
Recently read that they’re finding more and more subterranean animals (admittedly, small thing like roundworms) in deep rock, leading them to believe there are probably a lot more of them that we don’t know about.
The implication of the OP is “land animal,” but it should be noted that many invertebrates and a few families of fish live at the floor of the deep oceans, well below anywhere that humans have actually gotten to underground (as opposed to boreholes).
Aren’t there some worms or something living on the sulfur compounds where the ocean floor is spreading?
Once, for twenty minutes…
Our ecology is based, ultimately, on photosynthesis. This source of energy obviously isn’t available to deep-subterranean organisms; could something have evolved that receives its energy from some other source?
There is a fair amount of life that is based on extracting energy from hydrogen sulfide. There are ecosystems based around the hydrogen sulfide coming from hot underwater vents. This is probably what thelabdude is thinking of. I also saw a TV show about bacteria in caves living off of hydrogen sulfide in the caves.
Does bacteria count? Archaea has been found 3 miles down (2 miles water, 1 mile rock) living off methane.
Double nitpick: Archae aren’t bacteria, and neither of them are animals. If one were to divide all terrestrial life into only three categories, there’d be one for the archae, one for the bacteria, and one for everything else, including all of the animals, plants, and fungi, and a great many one-celled things.
I knew that you guys would go off on the “the sea floor is part of earth, so let’s paddle this idea around.” Yet again, my incomplete specificity and the apparently eagle-eyed ability of Dopers to find loopholes 
obscured the point of the question:
Fauna (ok, I’m upset now, so I’ll add flora) that live or extend anywhere above sea level on land downwards.
Sailbird above has the right point.