The limits for other types of diving are interesting, too.

All of the different gases you use for SCUBA diving have a maximum operating depth. At high pressures, various gases become toxic or cause side effects. Oxygen has a limit somewhere in the 1.2-1.6 bar range, and it becomes toxic. For deep dives, you need a lower percentage of oxygen in the mixture. Nitrogen causes nitrogen narcosis at higher pressures, so for deep dives, you need a lower percentage of nitrogen in the mixture. You need to add something else if you want to lower both nitrogen and oxygen, and that’s helium. At high enough pressures, helium causes high-pressure nervous syndrome, a.k.a. helium tremors.

That’s normally the limit. Pretty much every gas is bad for you at these pressures. There is an experimental alternative, which is to use hydrogen gas for the deepest dives, but people are understandably cautious about breathing a mixture of oxygen and hydrogen.