In 1941 and 1942, minds at the highest levels of government were occupied with how to stop the Germans who, having abandoned their Operation Sealion scheme to invade Britain by water, were thought to be digging their way across the Channel instead.
One British scientist, who was convinced of the danger, conjured up the image of legions of stormtroopers massing beneath the Downs, ready to burst forth upon an unsuspecting country. He urged the creation of a chain of listening posts along the coast capable of detecting the sound of digging - a kind of underground radar.
A thick file of documents showing how seriously this was taken lies among the wartime papers of the Cabinet Office in the archives of the Public Record Office in London. It contains papers bearing the signatures of important wartime figures such as R A Butler, the Tory minister; Lord Hankey, Churchill's Paymaster General; and Sir Henry Dale, the President of the Royal Society. Another signature belongs to John Cairncross, a Cabinet Office official who is now alleged to have been a member of the Philby spy ring and may well have relayed every detail to Moscow.
It was Hankey, as chairman of a scientific committee advising the Chiefs of Staff, who first took up the matter, although he acknowledged that it might be a 'fool suggestion'.
'I do not pretend to consider it more than a remote possibility, but a Channel tunnel would be a very fine secret weapon for Hitler,' he wrote.
Over several months in 1941 a number of committees and experts produced sufficiently divergent views on the feasibility of such a tunnel to leave a nagging doubt. The Institution of Civil Engineers, for example, said its best guess was that such an enterprise would take at least 12 years, but others were not so sure.
R B Bourdillon, a scientist at (incongruously) the Medical Research Council, argued in detail that by employing new rotary drilling technology and a clever system of disposing of the extracted materials underwater, the job might be done much more quickly and without detection. His estimate was that 16 months would suffice.
A number of factors supported the likelihood of a German project, he said: '1: The suspiciously easy abandonment of the 1940 invasion plans. The Germans do not usually abandon major strategic objectives after one failure. 2: The statements recently made in Germany that when invasion comes it will be by a method that the English do not expect. 3: The insistence on engineering degrees for the higher ranks of German army officers.' And so on.
Warning that the Germans might already have completed their tunnel - this was the end of 1941 - Bourdillon spelled out what might be involved: 'It is assumed that the tunnel or tunnels would branch at the English coast into 12 or perhaps 20 separate tunnels of from one to 10 or more miles in length . . . each ending in a vertical lift shaft of 200-500ft . . .
'Each lift shaft would end in a short T-chamber cut out about 100ft below the crest of a downland ridge. A series of short horizontal adits would lead towards the side of the ridge, but stop 20ft short of the surface until after dark on the first evening of invasion, when they would be blown open by powder charges.'
These concealed chambers might be located anywhere in a broad arc from St Leonards to the Isle of Sheppey, and from them, at the appointed hour, would burst hordes of German soldiers and tanks, ready to take over the country.
How many? Bourdillon had an answer: 'Under peacetime conditions each tube of the proposed Anglo-French Channel tunnels (as suggested in 1930) was estimated to be capable of carrying both 30,000 passengers and 30,000 tons of goods in 20 hours . . . . Under war conditions this rate could probably be increased for a limited period.
'With regard to the lift-shafts, I assume each to be capable of taking one heavy tank or 25 men with a transit time of two minutes, and loading and unloading times of two minutes each. This gives a six- minute cycle with a capacity for 500 lifts, if everything went without hitch, of 12,000 tanks a day, or 300,000 men . . . .'
Before we laugh, it is worth reflecting that the origins of this idea bear some comparison with the origin of the wartime atomic bomb project. In that case, two refugee scientists in Birmingham alerted the government to the notion that such a weapon might be possible and that if it was, the Germans might be making it. Feasibility studies were ordered, and they ultimately led, by a circuitous route, to the American bombs dropped on Japan.
Bourdillon's views were dismissed by some, but Lord Hankey insisted that the matter should be brought to a conclusion, and R A Butler, who succeeded him as chairman of the relevant Cabinet Office committees at this time, agreed.
While the scientific argument went on, other efforts were devoted to the matter. 'Intelligence is doing what it can to keep an eye on the possibility,' noted one memo, and at the same time the RAF were ordered to photograph regularly the likely tunnel-head at Sangatte on the French side, where the modern tunnel ends.
Eventually, in May 1942, experiments were carried out at Abbot's Cliff, near Dover, to determine whether German tunnellers might be detected from the surface. Experts from the Department of the Director of Torpedoes and Mining placed monitoring devices on the ground, while in an old tunnel shaft below, various types of digging equipment were started up.
'Good. Keep me au fait,' R A Butler scribbled across a page when the results were reported. The experts concluded that by distributing 50 listening devices along the key 10 miles of Channel coast, and setting up a permanent station to monitor readings, the threat could be adequately met.
There the file ends, with nothing further on the installation of the listening devices along the Kent coast. But the 'fool suggestion' of May 1941 and its aftermath deserve to be remembered alongside all the other schemes and misadventures that will have preceded the opening of the Channel tunnel to passenger traffic next October.
Ulrike Jordan is a research student at the German Historical Institute in London.Reuse content