It varies depending on the dimensions of the dam, but dams can be and usually are very tough targets to destroy. In World War II the British proved that it could be done despite considerable difficulties and were the first to seriously develop the art of dam busting. The British used delayed-action bouncing bombs from Lancaster bombers, but fortunately for the Egyptians, advancements in weapons technology would enable them to target the Ethiopian dam in a less risky way. The best way for Egypt to knock out a standing dam is to use retarded and delayed-action bombs deployed from very low altitudes, or even better, delayed-action joint direct attack munitions deployed at medium altitude. The difficulty is that the bomb needs to be deployed at the very base of the dam, underwater, where the concussive effect and pressure wave is greatly amplified.