The Cambridge Dictionary defines a frogman as: "someone who swims or works underwater for a long time wearing breathing equipment, flippers, and usually a rubber suit". The Macmillan Dictionary definition is identical for British and American English as: "someone who does police or military work under water using special clothes and equipment". Collins English Dictionary gives "a swimmer equipped with a rubber suit, flippers, and breathing equipment for working underwater" as the British English definition, and "a person trained and equipped, as with a rubber suit and scuba apparatus, for underwater demolition, exploration, etc." as an American definition, citing Webster's New World College Dictionary, 4th Edition. Copyright 2010 by Houghton Mifflin Harcourt. They also indicate a trend of steadily decreasing usage from the mid 20th century to 2008, and the word may be obsolescent. The Dictionary of American Slang reports that colloquial meanings include a scuba diver, particularly a professional or military diver of WWII. Tactical diving is a branch of professional diving carried out by armed forces and tactical units. They may be divided into: These groups may overlap, and the same men may serve as assault divers and work divers, such as the Australian Clearance Diving Branch (RAN). The range of operations performed by these operatives includes:  Typically, a diver with closed circuit oxygen rebreathing equipment will stay within a depth limit of with limited deeper excursions to a maximum of because of the risk of seizure due to acute oxygen toxicity. The use of nitrox or mixed gas rebreathers can extend this depth range considerably, but this may be beyond the scope of operations, depending on the unit. US and UK forces use these official definitions for mission descriptors:  Anti-frogman techniques are security methods developed to protect watercraft, ports and installations, and other sensitive resources both in or nearby vulnerable waterways from potential threats or intrusions by frogmen. Frogmen on clandestine operations use rebreathers, as the bubbles released by open-circuit scuba would reveal them to surface lookouts and make a noise which hydrophones could easily detect. In ancient Roman and Greek times, etc., there were instances of men swimming or diving for combat, sometimes using a hollow plant stem or a long bone as a snorkel. Diving with snorkel is mentioned by Aristotle (4th century BC). The earliest descriptions of frogmen in war are found in Thukydides' "History of the Peloponnesian War".