▁cont ingen cies . ▁D ivers ▁prof ic ient ▁in ▁these ▁emer gency ▁dr ills ▁are ▁less ▁likely ▁to ▁be ▁over wh el med ▁by ▁the ▁circumstances ▁when ▁things ▁do ▁not ▁go ▁according ▁to ▁plan , ▁and ▁are ▁less ▁likely ▁to ▁pan ic . ▁ ▁Dep th ▁Techn ical ▁div es ▁may ▁be ▁defined ▁as ▁being ▁div es ▁deeper ▁than ▁about ▁ ▁or ▁div es ▁in ▁an ▁overhead ▁environment ▁with ▁no ▁direct ▁access ▁to ▁the ▁surface ▁or ▁natural ▁light . ▁Such ▁environments ▁may ▁include ▁fresh ▁and ▁salt water ▁c aves ▁and ▁the ▁inter iors ▁of ▁ship w re cks . ▁In ▁many ▁cases , ▁technical ▁div es ▁also ▁include ▁planned ▁decom pression ▁carried ▁out ▁over ▁a ▁number ▁of ▁stages ▁during ▁a ▁controlled ▁as cent ▁to ▁the ▁surface ▁at ▁the ▁end ▁of ▁the ▁d ive . ▁The ▁depth - based ▁definition ▁is ▁based ▁on ▁risk ▁caused ▁by ▁the ▁progress ive ▁imp air ment ▁of ▁mental ▁compet ence ▁with ▁increasing ▁partial ▁pressure ▁of ▁resp ired ▁nit ro gen . ▁Bre ath ing ▁air ▁under ▁pressure ▁causes ▁nit ro gen ▁nar cos is ▁that ▁usually ▁starts ▁to ▁become ▁a ▁problem ▁at ▁depth s ▁of ▁ ▁or ▁greater , ▁but ▁this ▁diff ers ▁between ▁divers . ▁In cre ased ▁depth ▁also ▁increases ▁the ▁partial ▁pressure ▁of ▁o xygen ▁and ▁so ▁increases ▁the ▁risk ▁of ▁o xygen ▁to x icity . ▁Techn ical ▁div ing ▁often ▁includes ▁the ▁use ▁of ▁breath ing ▁mi xt ures ▁other ▁than ▁air ▁to ▁reduce ▁these ▁ris ks , ▁and ▁the ▁additional ▁complexity