Sydney Knowles was a British frogman in and after WWII.

He was one of a group of underwater guard divers who checked for limpet mines in Gibraltar harbor during the period of Italian frogman and manned torpedo attacks.  They dived with Davis Escape Sets, which until then had not been used much if at all for swimming down from the surface.  At first they swam by breast stroke without swimfins.

On 10 September 1941, during one such attack, two of the Italian frogmen (Lt.Visintini and Petty Officer Magro) died, probably killed by depth charges.  Their bodies were recovered, and their swimfins were taken and from then on used by Sydney Knowles and Commander Lionel Crabb.

Lionel "Buster" Crabb OBE (28 January 1909-(?) 19 April 1956) was a British Royal Navy frogman who vanished during a reconnaissance mission around a Soviet cruiser in 1956.   Lionel Crabb was born in 1909 to Hugh and Beatrice Crabb of Streatham, South West London. They were a poor family.  In his youth he held many jobs but also joined the merchant navy.

At the outbreak of the World War II, Crabb was first an army gunner and then joined the Royal Navy in 1941.  The next year he was sent to Gibraltar where he worked in a mine and bomb disposal unit to remove the Italian limpet mines that their divers had attached to the hulls of Allied ships.  Initially Crabb just disarmed mines British divers removed, but eventually he decided to learn to dive himself.  He received a George Medal for his efforts and was promoted to Lieutenant Commander.  In 1943 he become Principal Diving Officer for Northern Italy, was assigned to clear mines in the ports of Livorno and Venice and was later created OBE for this duty.  By this time he had also gained a nickname; "Buster", after US swimmer Buster Crabbe.

After the war Crabb was stationed in Palestine and led an underwater explosives disposal team that removed mines placed by Jewish rebels.  After 1947, he was demobilized from the military.

Crabb moved to civilian job and used his diving skills to explore the wreck of a Spanish galleon and located a suitable site for a discharge pipe for the Atomic Weapons Research Establishment at Aldermaston.  He later returned to work for the Royal Navy.  He twice dived to investigate sunken Royal Navy submarines -- HMS Truculent in January 1950 and HMS Affray in 1951 -- to find out whether there were any survivors. Neither effort was successful, however. Crabb married Margaret Elaine Player in 1952 but they divorced after a few years.

In 1955 Crabb took frogman Sydney Knowles with him to investigate the hull of the Soviet ship Sverdlov to evaluate its greater manoeuvrability.  According to Knowles, they found a circular opening at the ship's bow and inside it a large propeller that could be directed to give thrust to the bow.  March 1955 Crabb was made to retire due to his age.

In 1956 however, MI6 recruited Crabb to investigate Soviet cruiser Ordzhonikidze, that had brought Nikita Khrushchev and Nikolai Bulganin on a diplomatic mission to Britain.   Reputedly he was asked to look for anti-sonar equipment and mine-laying hatches. In April 19, 1956 Crabb dived into Portsmouth Harbour and his MI6 controller never saw him again. Crabb's companion in the Sally Port Hotel took all his belongings and even the page of the hotel registry book where they had written their names.  Ten days later British newspapers published stories about Crabb's disappearance in an underwater mission.

MI6 tried to cover up this espionage mission.  On April 29 the Admiralty announced that Crabb has vanished when he had taken part of trials of secret underwater apparatus in Stokes Bay. Soviets answered by releasing a statement stating that the crew of Ordzhonikidze had seen a frogman near the cruiser on April 19.

British newspapers speculated that Soviets had captured Crabb and taken him to the Soviet Union. The British Prime Minister Anthony Eden apparently disapproved of the fact that MI6 had operated without his consent in the UK (the preserve of the Security Service, "MI5"), and forced director-general John Sinclair to resign.

On June 9, 1957, a body in a frogman suit was found floating off Pilsey Island. It was missing its head and both hands, which made it impossible to identify. Crabb's ex-wife was not sure enough to identify the body, nor was Crabb's girlfriend Pat Rose.  Sidney Knowles said that Crabb had had a similar scar on the left knee. An inquest jury returned an open verdict but the coroner announced that he was satisfied that the body was that of Lionel Crabb.

After Crabb's disappearance there have been numerous speculations of what happened to him. Members of Parliament J.S. Kerans in 1961 and Marcus Lipton in 1964 submitted proposals to reopen the case but were rebuffed.  Various people speculated that he had been killed by some secret Soviet underwater weapon; that he had been captured and imprisoned in Lefortovo prison with a prison number 147;  that he had been brainwashed to work for the Soviet Union to train their frogman teams; that he had defected and became a Commander in the Soviet Navy under the name Lev Lvovich Korablov; that he was in the Soviet Special Task Underwater Operational Command in the Black Sea Fleet; or that MI6 had asked him to defect so he could become a double agent.  In a 1990 interview Joseph Zwerkin, an ex-member of Soviet Naval intelligence who had moved to Israel after the fall of the Soviet Union, claimed that Soviets had noticed Crabb in the water and that a Soviet sniper had shot him. Official government documents regarding Crabb's disappearance are not scheduled to be released until 2057.

On March 26, 2006, The Mail On Sunday published an article by Tim Binding entitled Buster Crabb was murdered - by MI5. Binding wrote a fictionalised account of Crabb's life, Man Overboard, published in 2005. Binding states that, following the publication, he was contacted by Sydney Knowles, now living in Málaga, Spain. Binding alleges he then met Knowles in Spain and was told that Crabb was known by MI5 to have intentions of defecting to the USSR. This would have been embarrassing for the UK, Crabb being an acknowledged war hero. Knowles suggests that MI5 set up the mission to the Ordzhonikidze specifically to murder Crabb, and supplied Crabb with a new diving partner ordered to kill him. Binding states that Knowles alleges that he was ordered by MI5 to identify the body found as Crabb, when he knew it was definitely not Crabb. Knowles went along with the deception. Knowles also alleges that his life was threatened in Torremolinos in 1989, at a time when Knowles was in discussions with a biographer