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. A seismic bomb differs somewhat in concept from traditional bombs, which usually explode at or near the surface, and destroy their target directly by explosive force. In contrast, a seismic bomb is dropped from high altitude to attain very high speed as it falls and upon impact, penetrates and explodes deep underground, causing massive caverns or craters known as camouflets, as well as intense shockwaves. In this way, the seismic bomb can affect targets that are too massive to be affected by a conventional bomb, as well as damage or destroy difficult targets such as bridges and viaducts.
Earthquake bombs were used towards the end of World War II on massively reinforced installations, such as submarine pens with concrete walls several metres thick, underground caverns, buried tunnels, and bridges.
During development Barnes Wallis theorised that a highly aerodynamic, very heavy bomb with a delayed detonation would cause damage to a target through shock waves travelling through the ground, hence the nickname earthquake bombs.
The airmen who dropped the bombs reported that the target structures stood undamaged by the detonation; "But then the crater collapsed, the ground shifted and the target collapsed". Later computer simulations reached the same conclusions; the significant part of the damage was done by generating a cavity in the ground, and that cavity collapsing in on itself causing the ground to shift and the target's foundation to shift or break causing catastrophic structural damage to the target supported by said foundation. The shifting ground caused any larger structure to become severely damaged, even if the bomb missed the target but created a crater near it.
They were not true seismic weapons, but effective cratering weapons.