Final answer:
Helium is the most common inert gas used in place of nitrogen in deep-sea diving to avoid nitrogen narcosis and decompression sickness.
Step-by-step explanation:
The most common inert gas used by deep-sea divers as a substitute for nitrogen is helium. This is done to avoid nitrogen narcosis, a condition where dissolved nitrogen in the blood and tissues causes a state of disorientation due to the pressures experienced at depth. For deep dives, divers often use a breathing mix known as heliox, which is a mixture of oxygen and helium. Helium is preferred due to its non-narcotic properties at high pressures, making it a safer option compared to nitrogen. Furthermore, the low solubility of helium in human tissues compared to nitrogen reduces the risk of decompression sickness, or "the bends".