Wallis realized that the force of an explosion is quickly diminished in a compressible medium. In the compressible medium of air, the explosive power diminishes rapidly with distance. Due to a 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.
Though 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 bomb proof critical installations such as bunkers.
If a bomb could be designed to explode in water, soil, or other solid materials, the explosive force would carry much further, and be transmitted more efficiently to the target object.
Wallis' idea was to drop a large heavy bomb with a hard armoured tip at supersonic speed so that it penetrated the ground. This effect would be rather like 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, and the resulting shock wave would produce the equivalent of a miniature earthquake, destroying any nearby structures such as dams, railways, viaducts, etc. Any concrete strengthening of the target would just serve to enclose the force better.
Wallis foresaw that German industry would have to be disrupted, 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's ideas were shown to be superbly successful.
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 have been dropped from 40,000 feet (12 km). At this time the RAF had no aircraft capable of carrying a ten-ton bombload let alone lifting it to such a height. Wallis designed a six-engine aeroplane for the task, called the "Victory Bomber", but was not taken seriously by the military top brass of the day.
After he developed the bouncing bomb, however, RAF Bomber Command were prepared to listen to his ideas, even though they often sounded very strange to them. The officer classes of even 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 “earth quake bomb concept” such as the Tallboy and Grand Slam, although these were never dropped from more than about 25,000 feet (8 km). Even from this low height, the earth quake 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 the U-Boats in their 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 through a mountain. It was dropped on the mountain, drilled straight through the rock, and exploded in the tunnel below. The result was that the complete line remained unusable until the end of the war.
After World War II the United States developed the 43,600 lb (19,800 kg) T12 demolition bomb that was designed to create an earth quake effect. There was little or no development of conventional deep penetrating bombs until the first Gulf War given the availability of nuclear weapons. During the first Gulf War, however, the need for a conventional deep penetrator became clear. In three weeks a cooperative effort directed by the Armament Systems Division at Eglin AFB, Florida, developed the 5,000 lb (2,300 kg) GBU-28 that was used successfully against a deep underground complex not far from Baghdad just before the end of the war.