6 was the code name of a military trench-digging machine developed by the British Royal Navy at the beginning of World War II.

The machine was originally known as White Rabbit Number Six; this code name was never officially recognised, but it was said to be derived from Churchill's metaphorical ability to pull ideas out of a hat.

[4] The machine's development and production was enthusiastically backed by Winston Churchill and work on it continued well past the point when there was no obvious use for it.

When Churchill had served as the First Lord of the Admiralty in the First World War and he had been largely responsible for the establishment of the Royal Navy's Landships Committee, which sponsored experiments with armoured tractors that eventually resulted in the invention of the tank.

[8] The machine he envisioned would be capable of breaking the stalemate of trench warfare that had developed during World War I, and would thereby avoid the atrocious conditions and high casualty rate that resulted.

Correctly estimating the power required to perform this feat was essential to the success of the project, but the nearest comparable machine the designers could take figures from were the giant bucket and chain excavators used in Germany for open cast mining of lignite.

[13] Initial designs envisaged a large circular cutter the diameter of the trench to be dug and operating in the manner of modern tunnel boring machines.

[15] On 6 December 1939, Churchill was told that Ruston-Bucyrus would be able to build 200 trench-cutting machines by March 1941, and they proposed a wider version that would produce a trench in which tanks could drive.

The model together with its accessories was packed into a mahogany box resembling a coffin; as it was carried to the station in Bath, many bystanders respectfully bowed their heads.

[19] The demonstration went so well that Churchill's smile of pleasure "almost dislodged his cigar" and he ordered that a further demonstration should be arranged for that evening, to which Churchill was accompanied by the prime minister, Neville Chamberlain, the Chancellor of the Exchequer Sir John Simon and the Chief of the Imperial General Staff Sir Edmund Ironside.

[20] On 7 February 1940 the government gave approval for the construction of 200 narrow "infantry" and 40 wider "officer" machines, the latter creating a trench wide enough for tanks.

[6] In the following weeks, the Germans noticed intense patrol activity in front of the Siegfried Line as the French collected soil samples so that technicians could determine the most suitable places for the Cultivators to advance.

[19] However, the production of Cultivator almost immediately faced a problem as the Air Ministry reserved for RAF use all the Rolls-Royce Merlin engines being produced.

They called in Sir Harry Ricardo, who suggested using a pair of 600 hp lightweight diesel engines built by Davey, Paxman and Co.[19][21][22] The change meant a great deal of redesign work, but the new arrangement had some advantages.

[24] The body was driven by two tracks that were 2 feet (0.61 m) wide, and on the surface steering was possible by means of dog clutches on the gear-box output shafts.

When digging, only small changes in direction were possible by means of hydraulically operated steering doors, one on each side of the machine.

[21] There was an arrangement to draw an adjustable amount of soil back into the trench and under the vehicle's tracks so as to counter any tendency to heel over.

[25] In April 1940, there came a huge surprise: someone else had invented a high-speed trenching machine envisaged to be used in a similar way but working by quite different means.

Clarke had prepared a paper "A Consideration of New Offensive Means" and followed this with "Notes on Design of Trench Forming Machines" for the Royal Engineers.

[26] Clarke accepted a job as a temporary civil servant; he was hired as an assistant director of NLE with a salary of £1,000 per annum.

[27][26] Clarke's idea was to use an armoured vehicle of some sort equipped with a hydraulic ram to insert explosive charges into the ground ahead.

[26] Clarke's machine would require thick armour to protect itself from its own explosions at the front and from the possibility of attack from the rear.

[28]The number of units was soon even more significantly reduced to just 33 machines[29] and by July Churchill was finding other tasks to assign to Mr Hopkins.

Historian John Turner attributes this dogged persistence to Churchill's failure to get his mole accepted during the First World War.