Question about diving?

Alright im wondering, After how many meters is diving dangerous? For example i heard there are many problems that could occur from diving (EG. i saw someone get paralyzed from ascending too fast) what are they? I dove 5 meters my ears felt like they were about to explode O_O i had to go back up. I didnt use any gear though just dove (yea i can hold my breathe for a LONG time)