The US Air Force's BLU-113 "bunker buster" warhead was originally developed in a hurry during the 1991 Gulf War to attack fortified Iraqi command centres deep underground.
It is a 4,500 lb (2,041 kg) penetrating warhead, later used in Serbia and in Afghanistan, and now in the Iraqi capital, Baghdad.
It is carried in the form of the GBU-28 laser-guided bomb by the B-2 stealth bomber and F-15E fighter-bomber, or the GBU-37 satellite-guided bomb by the B-2 alone.
Satellite guidance means the bomb can be used even through cloud or smoke - because laser guidance, though highly accurate, is degraded by those.
The warhead includes more than 600 lb (272 kg) of high explosive - the rest is believed to be dense depleted uranium.
It is triggered by a special "hard target smart fuse", manufactured under the slogan "No Place to Hide".
B-2 bomber taking off to attack Iraq
The fuse is a delayed-action device that can be pre-programmed to go off after encountering a certain number of hard layers or an air space, or at a particular depth.
A BBC correspondent in Baghdad described how the building he was in shook for several seconds when a bunker buster bomb landed some distance away.
Defence analysts say the US armed forces have been increasingly interested in developing a range of weapons to hit deeply buried targets.
The Iraqis are known to have drawn on expertise from the former Yugoslavia in constructing underground centres.
Bunker busters were developed extremely quickly during the air campaign in the Gulf War in 1991 after it became clear that existing weapons were proving ineffective against underground targets.
The GBU-28 was not even in the planning stages when Kuwait was invaded in 1990.
The US Air Force asked for ideas a week after military operations started.
The first bunker buster was built on 1 February 1991 using surplus 8-inch artillery tubes as the main part of the weapon. The project received the official go-ahead a fortnight later.
Initial development and testing proved that the bombs could penetrate more than 20 ft of concrete, while a flight test demonstrated the bomb's ability to penetrate more than 100 ft of earth.
The first two operational bombs were delivered to the Gulf on 27 February and used during Operation Desert Storm.