The gas element used in deep-sea diving?

Answer 1

Deep-sea diving act of descending into deep water, with some breathing apparatus, help them to remain there for an extended period. There can be multiple gases used depending on desired depth and length of dive the common is a tri-mixed gas of Nitrogen, Oxygen, and Helium.

Answer 2

Normal divers would use compressed air, however for deep sea diving a large proportion of the Nitrogen in compressed air is replaced with Helium, this is done to prevent nitrogen bubbles forming in the blood as the diver decompresses, this causes the bends and can be fatal, so the answer to your question is Helium.

Answer 3

normal gas for Scuba is 20% oxygen, 80% nitrogen.

You can take a nitrox course, in which you are then qualified to use different blends of oxygen and nitrogen. i.e. 40% oxygen and 60% nitrogen