most commercial diving uses surface-supplied diving equipment for the main gas supply when this is practicable. Surface supplied divers may be required to carry scuba as an emergency breathing gas supply to get them to safety in the event of a failure of surface gas supply. There are divers who work, full or part-time, in the recreational diving community as instructors, assistant instructors, divemasters and dive guides. In some jurisdictions the professional nature, with particular reference to responsibility for health and safety of the clients, of recreational diver instruction, dive leadership for reward and dive guiding is recognised and regulated by national legislation. Other specialist areas of scuba diving include military diving, with a long history of military frogmen in various roles. Their roles include direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations. In civilian operations, many police forces operate police diving teams to perform "search and recovery" or "search and rescue" operations and to assist with the detection of crime which may involve bodies of water. In some cases search and rescue diving teams may also be part of a fire department, paramedical service or lifeguard unit, and may be classed as public service diving. There are also professional divers involved with the underwater environment, such as underwater photographers or underwater videographers, who document the underwater world, or scientific diving, including marine biology, geology, hydrology, oceanography and underwater archaeology. The choice between scuba and surface supplied diving equipment is based on both legal and logistical constraints. Where the diver requires mobility and a large range of movement, scuba is usually the choice if safety and legal constraints allow. Higher risk work, particularly in commercial diving, may be restricted to surface supplied equipment by legislation and codes of practice. Alternatives to scuba for diving There are alternative methods that a person can use to survive and function while underwater, currently including: free-diving – swimming underwater on a single breath of air. snorkeling – a form of free-diving where the diver's mouth and nose can remain underwater when breathing, because the diver is able to breathe at the surface through a short tube known as a snorkel. surface-supplied diving – originally, and still used in professional diving for long or deep dives, where an umbilical cable provides breathing gas, voice communication and sometimes warm water to heat the diving suit from