Deep diving has different meanings depending on the context. Even in recreational diving the meaning may vary: In recreational diving, a depth below about 30 metres (98 ft), where nitrogen narcosis becomes a significant hazard for some divers, may be considered a "deep dive". In technical diving, a depth below about 60 metres (200 ft) where hypoxic breathing gas becomes necessary to avoid oxygen toxicity may be considered a "deep dive". In professional diving, a depth that requires special equipment, procedures, or advanced training may be considered a deep dive.