Frogman
scuba unit. This was a revolutionary alternative way to dive, and the start of the transition from the usual heavy underwater diving equipment of the hard hat divers which had been in general use since the 18th century, to self-contained divers, free of being tethered by an air line and rope connection.

Wartime operations: After Italy declared war, the Decima Flottiglia MAS (Xª MAS) attempted several frogmen attacks on British naval bases in the Mediterranean between June 1940 and July 1941, but none were successful, because of equipment failure or early detection by British forces. On September 10, 1941, eight Xª MAS frogmen were inserted by submarine close to the British harbour at Gibraltar, where using human torpedoes to penetrate the defences, sank three merchant ships before escaping through neutral Spain. An even more successful attack, the Raid on Alexandria, was mounted on 19 December on the harbour at Alexandria, again using human torpedoes. The raid resulted in disabling the battleships HMS Queen Elizabeth and HMS Valiant together with a destroyer and an oil tanker, but all six frogmen were captured. Frogmen were deployed by stealth in Algeciras, Spain, from where they launched a number of limpet-mine attacks on Allied shipping at anchor off Gibraltar. Some time later they refitted the interned Italian tanker Olterra as a mothership for human torpedoes, carrying out three assaults on ships at Gibraltar between late 1942 and early 1943, sinking six of them.
Nazi Germany raised a number of frogmen units under the auspices of both the Kriegsmarine and the Abwehr, often relying on Italian expertise and equipment. In June 1944, a K-Verband frogman unit failed to destroy the bridge at Bénouville, now known as Pegasus Bridge, during the Battle of Normandy. In March 1945, a frogman squad from the Brandenburgers was deployed from their base in Venice to destroy the Ludendorff Bridge over the Rhine which had been captured by the US Army in the Battle of Remagen. Seven frogmen swam 17 kilometres (11 mi) downriver to the bridge carrying explosives, but were spotted by Canal Defence Lights. Four died, two from hypothermia, and the rest were captured.
The British Royal Navy had captured an Italian human torpedo during a failed attack on Malta; they developed a copy called the Chariot and formed a unit called the Experimental Submarine Flotilla, which later merged with the Special Boat Service. A number of Chariot operations were attempted, most notably Operation Title in October 1942, an attack on the German battleship Tirpitz, which had to be abandoned when a storm hit the fishing boat which was towing the Chariots into position. Operation Principal in January 1943 was an attack by eight Chariots on La Maddalena and Palermo harbours; although all the Chariots were lost, the new Italian cruiser Ulpio Traiano was sunk. The last and most successful British operation resulted in sinking two liners in Phuket harbour in Thailand in October 1944. Royal Navy divers did not use fins until December 1942.

Wartime developments: In 1933 Italian companies were already producing underwater oxygen rebreathers, but the first diving set known as SCUBA was invented in 1939 by Christian Lambertsen, who originally called it the Lambertsen Amphibious Respirator Unit (LARU) and patented it in 1940. He later renamed it the Self Contained Underwater Breathing Apparatus, which, contracted to SCUBA, eventually became the generic term for both open circuit and rebreather autonomous underwater breathing equipment.
Lambertsen demonstrated it to the Office of Strategic Services (OSS) (after already being rejected by the U.S. Navy) in a pool at a hotel in Washington D.C. OSS not only bought into the concept, they hired Lambertsen to lead the program and build up the dive element of their Maritime Unit. The OSS was the predecessor of the Central Intelligence Agency; the maritime element still exists inside the CIA's Special Activities Division.
John Spence, an enlisted member of the U.S. Navy, was the first man selected to join the OSS group.

Postwar operations: In April 1956, Commander Lionel Crabb, a wartime pioneer of Royal Navy combat diving, disappeared during a covert inspection of the hull of the Soviet Navy Sverdlov-class cruiser, Ordzhonikidze, while she was moored in Portsmouth Harbour.
The Shayetet 13 commandos of the Israeli Navy have carried out a number of underwater raids on harbors. They were initially trained by veterans of Xª MAS and used Italian equipment. As part of Operation Raviv in 1969, eight frogmen used two human torpedoes to enter Ras Sadat naval base near Suez, where they destroyed two motor torpedo boats with mines.
During the 1982 Falklands War, the Argentinian Naval Intelligence Service planned an attack on British warships at Gibraltar. Code named Operation Algeciras, three frogmen, recruited from a former anti-government insurgent group, were to plant mines on the ships' hulls. The operation was abandoned when the divers were arrested by Spanish police and deported.
In 1985, the French nuclear weapons tests at Moruroa in the Pacific Ocean was being contested by environmental protesters led by the Greenpeace campaign ship, Rainbow Warrior. The Action Division of the French Directorate-General for External Security devised a plan to sink the Rainbow Warrior while it was berthed in harbor at Auckland in New Zealand. Two divers from the Division posed as tourists and attached two limpet mines to the ship's hull; the resulting explosion sank the ship and killed a Netherlands citizen on board. Two agents from the team, but not the divers, were arrested by the New Zealand Police and later convicted of manslaughter. The French government finally admitted responsibility two months later.
In the U.S. Navy, frogmen were officially phased out in 1983 and all active duty frogmen were transferred to SEAL units. In 1989, during the U.S. invasion of Panama, a team of four U.S. Navy SEALs using rebreathers conducted a combat swimmer attack on the Presidente Porras, a gunboat and yacht belonging to Manuel Noriega. The commandos attached explosives to the vessel as it was tied to a pier in the Panama Canal, escaping only after being attacked with grenades. Three years later during Operation Restore Hope, members of SEAL Team One swam to shore in Somalia to measure beach composition, water depth, and shore gradient ahead of a Marine landing. The mission resulted in several of the SEALs becoming ill as Somalia's waters were contaminated with raw sewage.
In 1978, the U.S. Navy Special Operations Officer (1140) community was established by combining Explosive Ordnance Disposal (EOD) and Expendable Ordnance Management officers with Diving and Salvage officers.  Special Ops Officers would become qualified in at lease two functional areas - normally EOD or Diving and Salvage, and Expendable Ordnance management. Officers trained in diving and salvage techniques were now allowed to follow a career pattern that took advantage of their training, and Unrestricted line officers were now permitted to specialize in salvage, with repeat tours of duty, and advanced training. Career patterns were developed to ensure that officers assigned to command were seasoned in salvage operations and well qualified in the technical aspects of their trade. "The combination gave a breadth and depth of professionalism to Navy salvage that had not been possible before."

Gallery: 

See also: List of military diving units
Lionel Crabb – Royal Navy frogman and MI6 diver
Military diving
Underwater Demolition Team

References: 

Further reading: Frogman operations: Decima Flottiglia MAS, Underwater Demolition Team, human torpedo, Sinking of the Rainbow Warrior, Russian commando frogmen
Bush, Elizabeth Kauffman (2004). America's first frogman : the Draper Kauffman story. Naval Institute Press. ISBN 1-59114-098-6. OCLC 55699399.
Fraser, Ian (1957). Frogman V.C. Angus & Robertson. OCLC 1599838.
Pugh, Marshal (1956). Frogman: Commander Crabb's story. OCLC 1280137.
Welham, Michael G.; Welham, Jacqui (1990). Frogman Spy: the mysterious disappearance of Commander 'Buster' Crabb. W.H. Allen. ISBN 1-85227-138-8. OCLC 21979335.
Tony Groom: DIVER. Royal Naval Clearance Divers work in the Falklands conflict. ISBN 978-1574092691.

External links:  Media related to Frogman at Wikimedia Commons
Panerai during World War Two Archived 2019-04-14 at the Wayback Machine
Frogman - Training, Equipment, and Operations of Our Navy's Undersea Fighters - C.B. Colby Archived 2019-03-09 at the Wayback Machine
List of books about frogmen