risks through proper procedures and appropriate equipment. The requisite skills are acquired by training and education, and honed by practice. Open-water certification programmes highlight diving physiology, safe diving practices, and diving hazards, but do not provide the diver with sufficient practice to become truly adept. Scuba divers by definition carry their breathing gas supply with them during the dive, and this limited quantity must get them back to the surface safely. Pre-dive planning of appropriate gas supply for the intended dive profile lets the diver allow for sufficient breathing gas for the planned dive and contingencies. They are not connected to a surface control point by an umbilical, such as surface-supplied divers use, and the freedom of movement that this allows, also allows the diver to penetrate overhead environments in ice diving, cave diving and wreck diving to the extent that the diver may lose their way and be unable to find the way out. This problem is exacerbated by the limited breathing gas supply, which gives a limited amount of time before the diver will drown if unable to surface. The standard procedure for managing this risk is to lay a continuous guideline from open water, which allows the diver to be sure of the route to the surface. Most scuba diving, particularly recreational scuba, uses a breathing gas supply mouthpiece that is gripped by the diver's teeth, and which can be dislodged relatively easily by impact. This is generally easily rectified unless the diver is incapacitated, and the associated skills are part of entry-level training. The problem becomes severe and immediately life-threatening if the diver loses both consciousness and the mouthpiece. Rebreather mouthpieces that are open when out of the mouth may let in water which can flood the loop, making them unable to deliver breathing gas, and will lose buoyancy as the gas escapes, thus putting the diver in a situation of two simultaneous life-threatening problems. Skills to manage this situation are a necessary part of training for the specific configuration. Full-face masks reduce these risks and are generally preferred for professional scuba diving, but can make emergency gas sharing difficult, and are less popular with recreational divers who often rely on gas sharing with a buddy as their breathing gas redundancy option. Risk The risk of dying during recreational, scientific or commercial diving is small, and on scuba, deaths are usually associated with poor gas management,