Information:  - Scuba diving is a mode of underwater diving in which the scuba diver uses a self-contained underwater breathing apparatus (scuba) which is completely independent of surface supply, to breathe underwater. Scuba provides the diver with the advantages of mobility and horizontal range far beyond the reach of an umbilical hose attached to surface-supplied diving equipment (SSDE). Scuba divers engaged in armed forces covert operations may be referred to as frogmen, combat divers or attack swimmers.  - Technical diving (also referred to as tec diving or tech diving) is scuba diving that exceeds the agency-specified limits of recreational diving for non-professional purposes. Technical diving may expose the diver to hazards beyond those normally associated with recreational diving, and to greater risk of serious injury or death. The risk may be reduced by appropriate skills, knowledge and experience, and by using suitable equipment and procedures. The skills may be developed through appropriate specialised training and experience. The equipment often involves breathing gases other than air or standard nitrox mixtures, and multiple gas sources.  - Nitrox is used to a lesser extent in surface-supplied diving, as these advantages are reduced by the more complex logistical requirements for nitrox compared to the use of simple low-pressure compressors for breathing gas supply. Nitrox can also be used in hyperbaric treatment of decompression illness, usually at pressures where pure oxygen would be hazardous. Nitrox is not a safer gas than compressed air in all respects; although its use can reduce the risk of decompression sickness, it increases the risk of oxygen toxicity and fire.  - Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment . In some diving circles , the term `` recreational diving '' is used in contradistinction to `` technical diving '' , a riskier and more demanding application of the sport which requires greater levels of training , experience and equipment .  - A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common, and only natural, breathing gas - but a range of pure gases or mixtures of gases are used in breathing equipment and enclosed habitats such as scuba equipment, surface supplied diving equipment, recompression chambers, submarines, space suits, spacecraft, medical life support and first aid equipment, high-altitude mountaineering and anaesthetic machines.    Given the information, choose the subject and object entities that have the relation of 'subclass of'.
A:
recreational diving , underwater diving