The al-Qaida hide-outs at Tora Bora are a vast complex of natural caves fortified by concrete and steel, making it tough for bombs to reach fighters hunkered deep within, an expert on Afghan caves said Tuesday.

While bombs could penetrate much of the solid rock, mostly granite and crystalline stones, they may not reach some inner bunkers, said Jack Shroder, a professor of geography and geology at the University of Nebraska, who led a project to map Afghanistan in the 1970s.

Bombs don't need to penetrate to the deepest caves. All they need to do is to seal up all the entrances so that those trapped inside can't tunnel their way out. And by definition at least some part of entrances will be near the surface and vulnerable to penetrating bombs.

Shroder said an effective way to drive out people hiding in the caves might be the use of gas to render occupants unconscious.

He's working too hard. First, most of the potential anesthetic agents which could be used are also explosive (things like cyclohexane). Second, they require high dosage to work, and wear off in minutes. Third, any troops invading to take advantage of it would have to work with air tanks; simple gas masks would not offer protection. Anesthetic gases are not an effective weapon for something like this. If we have the ability to inject compounds into the air supply, it would be far better be to use a stink bomb. Compounds have been created which are repulsive and which we can smell in extremely small quantities; putting a few hundred kilograms of such a compound into an air intake would make the whole compound uninhabitable for months. One choice would be butyl mercaptan (the active ingredient that a skunk shoots) but the problem with it is that it is higly flammable. I know that much work has gone into the development of putrid agents for use in non-lethal weapons for police action; I have no doubt at all that there are may such things which could be used which are relatively non-toxic and non-flammable but which are impossible to be around. If friendly forces have access to air intakes for a bunker, that bunker won't be occupied for long. (discuss)