I quick little story of diving air 30 years ago. Keep in mind I do not condone diving deep on air if there are other options, but also remember deep is a relative term.
I once worked as a dive guide in Hawaii in 1980 - 1989; there were not a lot of rules or knowledge for that matter then
early on we simply used a Hawaiian backpack to hold our tank, this was a metal clamp device that fit over your shoulder, no BCD, we did not have an octopus or many of the other things like computers etc. we memorized the dive tables because we did not plan in advance how deep we would limit ourselves to....60/60, 70/50, 80/40.... Etc.
We dove Molokini every day when there were only two other dive boats going there, and we dove the back side as often as possible which is a shear wall which goes down to 360 feet.
Being young and enjoying having a bit of a buzz on we would do a dive just to get narced. beings that we were doing a couple 100 foot plus dives every day with tourist, then afternoon dives and then maybe a course, then after work or after a night out at the Sheraton disco a 2am dive for lobster. Needless to say we were doing a lot of diving daily and our tolerance was building.
so back to chasing the narc as we called it, we were going deeper and deeper to get good and narced, we would laugh at the other guy who was worse off than us, there were a lot of antics, once we had a girlfriend at around two hunderds who had really long hair and an octopus landed on her head and started attacking her head, we were totally narced out and trippin on this it was like some kind of fun drug that there was no hang over or anything as soon as you surfaced.
Well the deepest we ever went was 240, and it was not to say we went that deep it was because thats what it took to get us euphoric. Once we where cruising with this giant manta ray at somewhere around 200 - 220 it was like flying in outer space on an alien space ship.
Anyways I only pass this on as a story of diving deep, not smart, we were young, and looking for fun...narcosis is probably what kept our little group of divers from ever doing drugs or drinking to the point of toxicity....
I always used a mantra "Bubbles mean up, bubbles mean up, bubbles mean up" I would repeat this over and over in my head, and on more than one occasion I wasnt sure what it meant or why I was saying it, I just followed the bubbles until I understood
This all sounds so crazy now...but it is what it is