Final answer:
The deepest living underground organism is likely a type of bacterium that thrives in high-temperature and pressure environments such as the abyssal zone of the ocean, where life is sustained by chemosynthesis, not photosynthesis.
Step-by-step explanation:
The deepest living underground organism is likely a type of bacterium, such as Acetothermus paucivorans, which is not only a thermophile thriving in high temperatures but also a deeply branching bacterium. These organisms can exist in environments such as the abyssal zone, at depths of 4000 meters or greater, where conditions are extreme with very high pressure, very low to no oxygen content, and high nutrient content from decaying material. Life in these depths is sustained not by photosynthesis but by chemosynthesis, where bacteria like chemoautotrophs use minerals such as hydrogen sulfide from hydrothermal vents as an energy source. Additionally, other organisms mentioned that inhabit deep-sea environments include the tube worms, which live around hydrothermal vents in the deep ocean floor, and the Pompeii worm, notable for its heat tolerance and symbiotic relationship with bacteria.