ers ▁which ▁are ▁stored ▁by ▁sc uba ▁divers ▁along ▁the ▁return ▁route ▁containing ▁decom pression ▁and ▁emer gency ▁gas . ▁This ▁is ▁only ▁pract ic able ▁where ▁the ▁return ▁route ▁is ▁known ▁and ▁marked ▁by ▁a ▁guid eline . ▁Similar ▁cyl ind ers ▁are ▁carried ▁by ▁the ▁divers ▁when ▁the ▁route ▁back ▁is ▁not ▁secure . ▁They ▁are ▁commonly ▁mounted ▁as ▁s ling ▁cyl ind ers , ▁cli pped ▁to ▁D - r ings ▁at ▁the ▁sides ▁of ▁the ▁diver ' s ▁har ness . ▁The ▁divers ▁must ▁avoid ▁breath ing ▁o xygen ▁en rich ed ▁" de co ▁gas " ▁at ▁excess ive ▁depth s ▁because ▁of ▁the ▁high ▁risk ▁of ▁o xygen ▁to x icity . ▁To ▁prevent ▁this ▁happening , ▁cyl ind ers ▁containing ▁o xygen - rich ▁g ases ▁must ▁always ▁be ▁posit ively ▁ident ifiable . ▁One ▁way ▁of ▁doing ▁this ▁is ▁by ▁mark ing ▁them ▁with ▁their ▁maximum ▁operating ▁depth ▁as ▁clearly ▁as ▁possible . ▁ ▁Sur face ▁supplied ▁divers ▁may ▁be ▁supplied ▁with ▁a ▁gas ▁mixture ▁suitable ▁for ▁acceler ated ▁decom pression ▁by ▁connecting ▁a ▁supply ▁to ▁the ▁surface ▁gas ▁panel ▁and ▁providing ▁it ▁through ▁the ▁um bil ical ▁to ▁the ▁divers . ▁This ▁allows ▁acceler ated ▁decom pression , ▁usually ▁on ▁o xygen , ▁which ▁can ▁be ▁used ▁to ▁a ▁maximum ▁depth ▁of ▁ 3 0   ft ▁( 9   m ). ▁Sur face ▁supplied ▁hel io x ▁b ounce ▁divers ▁will ▁be ▁provided ▁with ▁mi xt ures ▁suitable ▁for ▁their ▁current ▁depth ,