When recreational Scuba Diving, you’ll typically use standard compressed air, that, with only 21% oxygen.
For various reasons though – longer diving time, less post-diving fatigue, shorter intervals between dives, it is possible to use what is known as ‘Enriched Air’ aka ‘Nitrox’ – that, air with a higher Oxygen content of 22-40%.
As your common John Citizen Scuba diver, you’ll need to do an approved course before you can suck down the good stuff, but hopefully you’ll walk away aware of the fact you can actually get Oxygen toxicity if you screw around, and after convulsing underwater, down to the bottomless depths you will sink.
I haven’t formally studied anything since my drivers licence 8 years ago, and even then I figured anything over 50% was a waste of effort, so asking to learn a 100 page manual along with calculations and tables was pushing it.
Nevertheless a week later and a perfect 100% on the exam, I had my ticket to extended Nemo terrorism.