breathing gas from each other in an emergency, and may carry an independent alternative air source if they do not choose to rely on a buddy. Divers may need to make an emergency ascent in the event of a loss of breathing gas which cannot be managed at depth. Controlled emergency ascents are almost always a consequence of loss of breathing gas, while uncontrolled ascents are usually the result of a buoyancy control failure. Other urgent emergencies may involve loss of control of depth and medical emergencies. Divers may be trained in procedures that have been approved by the training agencies for recovery of an unresponsive diver to the surface, where it might be possible to administer first aid. Not all recreational divers have this training as some agencies do not include it in entry-level training. Professional divers may be required by legislation or code of practice to have a standby diver at any diving operation, who is both competent and available to attempt rescue of a distressed diver. Two basic types of entrapment are significant hazards for scuba divers: Inability to navigate out of an enclosed space, and physical entrapment which prevents the diver from leaving a location. The first case can usually be avoided by staying out of enclosed spaces, and when the objective of the dive includes penetration of enclosed spaces, taking precautions such as the use of lights and guidelines, for which specialised training is provided in the standard procedures. The most common form of physical entrapment is getting snagged on ropes, lines or nets, and the use of a cutting implement is the standard method of dealing with the problem. The risk of entanglement can be reduced by careful configuration of equipment to minimise those parts which can easily be snagged, and allow easier disentanglement. Other forms of entrapment such as getting wedged into tight spaces can often be avoided, but must otherwise be dealt with as they happen. The assistance of a buddy may be helpful where possible. Scuba diving in relatively hazardous environments such as caves and wrecks, areas of strong water movement, relatively great depths, with decompression obligations, with equipment that has more complex failure modes, and with gases that are not safe to breathe at all depths of the dive require specialised safety and emergency procedures tailored to the specific hazards, and often specialised equipment. These conditions are generally associated with