It was March 1996. Libya had tunneled deep into a mountain south of Tripoli, allegedly to build a large chemical-weapons plant. To stop the project, the Clinton administration threatened to retaliate with "the whole range" of US weaponry up to and including a nuclear bomb.
Libyan leader Muammar Qaddafi apparently heeded the warning. Construction at the site halted, US intelligence showed.
Yet the Libyan complex was just one of more than 10,000 underground military facilities that have mushroomed in number during the 1990s in more than 70 countries from Iraq, Iran, and North Korea to Russia and China. Of those, more than 1,400 are known or suspected to be sheltering weapons of mass destruction (WMD), ballistic missiles, or military commands, according to US government reports.
To counter what the Pentagon considers a growing threat from such "hard and deeply buried targets," the Bush administration today is accelerating steps to develop more effective, earth-penetrating nuclear bombs.
The move which is gaining momentum with a push in Congress for funding in the 2003 budget comes charged with controversy. Challengers say the weapons are unnecessary, would still produce heavy collateral damage, and could spur nuclear proliferation if they are used to threaten non-nuclear states with preemptive strikes.
Attacking what he called the administration's "reckless new nuclear weapons policy," House Armed Service Committee member Rep. Tom Allen (D) of Maine said this month it could "ignite a nuclear arms race."
But Defense officials say that better bunker-busting nukes will provide vital deterrence against US foes. "The president ought to have options that enhance his ability ... to credibly threaten those targets," says a senior Pentagon official. "The obvious purpose of having a lower-yield weapon would be to ... make threatened use of that capability more credible and thereby to enhance deterrence."
The administration's immediate aim is to improve on its only existing earth-penetrating nuclear weapon, the 1,200-pound B61-11 gravity bomb. Entering the US arsenal in 1997, the B61-11 has an selectable yield of from 1 to 300 kilotons, nuclear experts say. But it can reach only a limited depth underground 10 to 20 feet in a dry lake bed in one government test and "cannot survive penetration into many types of terrain," according to the Pentagon's Nuclear Posture Review submitted to Congress in January.
A study is planned on the design and cost of a heavier, 5,000-pound modification of the B61-11, to see whether it could burrow deeper with its original warhead intact. Greater depth, in theory, would allow a lower-yield bomb to cause more underground destruction while also limiting nuclear fallout on the surface. Legislation moving through the House this week grants $15 million in the fiscal year 2003 budget to study such a "robust nuclear earth penetrator."
The current push is separate from failed efforts in previous years to develop so-called "mini-nukes." A 1994 law prohibits the nuclear laboratories from undertaking research and development that could lead to a precision weapon of less then 5 kilotons, because it would blur the distinction between nuclear and conventional war.
A successful study of the modified earth penetrator would pave the way for development and production. Yet just how "usable" and therefore credible as a deterrent such a bomb would be remains subject to doubt for both US officials and nuclear experts.
The Republican-led House Armed Services Committee, for example, points out in its report on the 2003 Defense bill that the earth penetrator "is not a new design, is not a low yield 'mini nuke', and is not 'clean' in the sense that fallout and collateral damage can be contained."
Even with an extremely tiny yield, scientists estimate that such a bomb would toss up an enormous cloud of debris and radioactive fallout.
"It would mean tens of thousands of people would be killed instead of hundreds of thousands" if used in a city such as Baghdad, says Rob Nelson, a physicist and researcher at the Program on Science and Global Security at Princeton University.
Other critics contend that conventional "bunker buster" bombs could effectively neutralize many underground targets. For example, the 5,000-pound GBU-28, used in Iraq during the Gulf War and more recently on caves in Afghanistan, has a new, steel cap that allows it to penetrate as deep or deeper than the B61-11.
Such conventional bombs could be used to seal the entrances of such complexes, entombing them, says Steve Schwartz, publisher of the Bulletin of the Atomic Scientists. "If we see them digging it out, we bomb it again," he says.
One major obstacle to both of these strategies whether conventional or nuclear is that US intelligence can't fully track the growing number of sophisticated underground facilities, many in so-called "rogue states," US military officials acknowledge.
Camouflaged and buried beneath the equivalent of from 70 to 300 feet of reinforced concrete, the facilities are built using either conventional drill-and-blast tunneling or more advanced mining technology, according to a Pentagon report to Congress last year.
"The challenges of hard and deeply buried targets [require] a much greater fidelity in intelligence than we currently possess," Adm. James Ellis told a Senate committee in March.