N2O is the name and it's damned fun. Time and space distortion, hallucinations and all. Trance music is unbelivable on N2O. Be sure it is N2O. NO2 is not something you would want in your system.
I heard it's really kickass--unfortunately you have to be deep underwater or in a pressure chamber as far as I know. I don't think there's any other way to achieve it.
From what I learned getting my openwater, you need to be down about 200ft, my instructor used to do it for fun, grab 2 tanks, drop to 220ft, hang around for 2 mins, head back up, incredibly dangerous really, you do some crazy shit stoned underwater, like worrying about a fish drowning and handing it your regulator. Anyway as far as I know it gives alcohol like effects, not hallucinations, you probably get the same effect from 1 litre of vodka.
I remember when my roof was made of spinning dots in psychedelic color, which were the "pixels" of the roof.... Crazy shit!! Gotta love it! My #1 drug...