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The earthquake bomb, or seismic bomb, was a concept that was invented by the British aeronautical engineer Barnes Wallis early in World War II and subsequently developed and used during the war against strategic targets in Europe.
An explosion in air does not transfer much energy into a solid, due to acoustic impedance. Due to the lack of accuracy of bombing in the face of anti-aircraft defences, air forces used area bombardment, dropping large numbers of bombs so that it would be likely that the target was hit. Although a direct hit from a light bomb would destroy an unprotected target, it was comparatively easy to armour ground targets with many yards of concrete, and thus render critical installations such as bunkers essentially bombproof. If a bomb could be designed to explode in water, soil, or other less compressible materials, the explosive force would be transmitted more efficiently to the target object.
Wallis's idea was to drop a large, heavy bomb with a hard armoured tip at supersonic speed so that it penetrated the ground — an effect comparable to a ten-ton bullet being fired straight down. It was then set to explode underground, ideally to the side of, or underneath a hardened target; the resulting shock wave would produce the equivalent of a miniature earthquake, destroying any nearby structures such as dams, railways, viaducts, etc. Any concrete reinforcement of the target would likely serve to enclose the force better.
Wallis also argued that, if the bomb penetrated deep enough, the explosion would not breach the surface of the ground and thus produce an underground cavern (a camouflet) which would remove the structure's underground support, thus causing it to collapse. The process was graphically described as a "trapdoor effect" or "hangman's drop".
Wallis foresaw that disrupting German industry would remove its ability to fight, and also understood that precision bombing was virtually impossible in the late 1930s. The technology for precision aiming was developed during World War II, however, and Barnes Wallis' ideas were then shown to be superbly successful, considering the standards at the time.
Wallis' first concept was for a ten-ton bomb that would explode some 130 ft (40 m) underground. To achieve this, the bomb would have had to be dropped from 40,000 feet (12 km). The RAF had no aircraft at the time capable of carrying a ten-ton bomb load aloft, let alone lifting it to such a height. Wallis designed a six-engine aeroplane for the task, called the "Victory Bomber", but he was not taken seriously by the military hierarchy of the day.
Wallis took a different line with attacks on Germany's industrial structure developing a means to destroy the supply of hydroelectric power. After he had developed the bouncing bomb and shown its possibilities, however, RAF Bomber Command were prepared to listen to his other ideas, even though they often thought them strange. The officer classes of the RAF at that time were often trained not in science or engineering, but in the classics, Roman and Greek history and language. They provided enough support to let him continue his research.
Later in the war, Barnes Wallis made bombs based on the “earthquake bomb concept”, such as the 6-ton Tallboy and then the 10-ton Grand Slam, although these were never dropped from more than about 25,000 feet (7.6 km). Even from this low height, the earthquake bomb had the ability to disrupt German industry while causing minimum civilian casualties. It was used to disable the V2 factory, bury the V3 guns, sink the Tirpitz and damage the U-boats' protective pens at St. Nazaire, as well as to attack many other targets which had been impossible to damage before. One of the most spectacular attacks was shortly after D-Day, when a Tallboy was used to prevent German tank reinforcements from moving by train. Rather than blow up the tracks — which would be repaired in a day or so — the bomb was targeted on a tunnel near Saumur which carried the line under a mountain. It was dropped on the mountain, drilled straight through the rock, and exploded in the tunnel below. As a result, the entire rail line remained unusable until the end of the war.
After World War II, the United States developed the 43,000 pounds (20,000 kg) T12 demolition bomb that was designed to create an earthquake effect. Given the availability of nuclear weapons, however, there was little or no development of conventional deep penetrating bombs until the 1991 Gulf War. During the Gulf War, the need for a conventional deep penetrator became clear. In three weeks, a cooperative effort directed by the Armament Systems Division at Eglin Air Force Base in Florida developed the 5,000 pounds (2,300 kg) GBU-28 that was used successfully by F-111Fs against a deep underground complex not far from Baghdad just before the end of the war.
The United States has developed a 30,000 pounds (14,000 kg) Massive Ordnance Penetrator, designed to attack very deeply buried targets without the use of nuclear weapons with the inherent huge levels of radioactive pollution and their attendant risk of retaliation in kind.