How to Destroy al Qaeda's Caves

Dec. 5 -- How, with 21st-century technology, do you fight a war against people who hide in caves?

That's the challenge for U.S. forces in Afghanistan, and the answer appears to be brute force combined with growing precision.

As anti-Taliban fighters came close to the mouths of al Qaeda's bunkers, American planes backed them up with so-called smart bombs — which have been modified so that they even work underground.

"The name of the game is trying to bring the explosive power of the warhead closer and closer to the intended target," says Clark Murdock of the Center for Strategic and International Studies. "And these intended targets are not just a hundred feet underground. They may be far deeper."

Indeed, by some estimates, the networks of caves in Afghanistan may reach 1,500 feet beneath the mountaintops in many places. Separate chambers have been dug in some for living quarters, weapons storage, and places for prayer.

But long before the conflict there, the U.S. military was working on weapons that could penetrate beneath the soil and still work with precision. Some date back 10 years or more, when the United States worried that Libya, Iraq and North Korea could be storing chemical or biological weapons deep in bunkers.

Smart Fuses

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A mainstay of the American arsenal is the GBU-28 (GBU for Guided Bomb Unit). It was rushed into action in the Persian Gulf War and has since been refined.

It can be dropped from a plane, and programmed to plow into the ground but not explode — not until its warhead has burrowed down to the deeply buried bunker it was intended to destroy. Equipped with what the Air Force calls a "Hard Target Smart Fuse," it should only go off in the part of the bunker a weapons officer specifies.

"We can actually sense when we're going through dirt, when we're going through reinforced concrete, or when we're going through free space," says Frank Robbins, director of the precision strike system program at Eglin Air Force Base in Florida, where the weapons are developed.