Guide to 'bunker-busting' bombs

The Guided Bomb Unit-28 (GBU-28), often known as a "bunker buster", was developed during the 1991 Gulf War for penetrating fortified Iraqi command centres deep underground.

These relatively simple, yet devastatingly effective weapons are now being used against underground positions in Afghanistan.

Carried by B-2 stealth bombers and F-15 fighters, the GBU-28 is a 5,000lb laser-guided, conventionally-armed bomb fitted with a 4,400lb penetrating warhead.

The operator illuminates a target with a laser and the bomb guides itself on to the mark.

Bunker busters can be delivered by B-2 stealth bombers

The BBC's defence correspondent Jonathan Marcus says the US armed forces have been increasingly interested in developing a new range of weapons to hit deeply buried targets.

"The need arose during the air campaign against Iraq, but strategic concerns in the Korean peninsula and elsewhere have added to the urgency of developing such systems," he said.

"The concern is driven by the fact that as the power of surveillance and satellite systems increase, so an enemy is likely to bury vital assets below ground.

Mountain missile bases

"The Americans, for example, believe that key elements of North Korea's nuclear programme may be underground. And the utility of cave networks and subterranan passages for groups like al-Qaeda is obvious."

The US Defence Secretary, Donald Rumsfeld, said in a media briefing on Thursday that "a lot of countries have done a lot of digging underground" - it was not unique to Afghanistan.

"It does make much more complicated the task of dealing with targets because, as you've known from photographs you've seen of North Korea, it is perfectly possible to dig into the side of a mountain and put a large ballistic missile in there and erect it and fire it out of the mountain from an underground post."

He said equipment such as that used to dig the Channel Tunnel could cut holes 50ft across and 200ft deep in a day.

Referring to the attacks in Afghanistan, he added: "You bet, to the extent we see a good deal of activity, a lot of so-called adits and tunnel entries and external indication of internal activity, we have targeted them."

Gulf War

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 bombs were originally carried by now-retired F-111 aircraft

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. The project received an official go-ahead a fortnight later.

Success

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.

A ground-support team prepares to attach a 'bunker buster'

The first operational bombs were delivered to the Gulf on 27 February. Only two bunker busters were dropped in Desert Storm, both by F-111 fighter-bombers.

One bomb hit its target, confirmed by the aircraft's onboard video camera which revealed smoke pouring from a bunker entrance about six seconds after impact.

After Operation Desert Storm, the Air Force made modifications and undertook further testing.

In 1997, the US spent $18.4m on producing more than 160 GBU-28s, a sign that the weapon had become an integral part of the US arsenal.

Although spurred by the campaign against Iraq, our correspondent says strategic concerns in the Korean peninsula and elsewhere have added to the urgency of developing such systems.

The Americans, for example, believe that key elements of North Korea's nuclear programme may be underground.