President Bush and Sen. John Kerry agree upon one thing - at least for the moment: The prolif eration of weapons of mass destruction (WMD), especially into the hands of terrorists, is the single greatest threat to American national security.

But the two quickly depart intellectual company over how to prevent proliferation. The difference is perhaps most stark over the issue of developing new, earth-penetrating weapons (EPW), affectionately known as "bunker-busters." (Kerry has voted to cancel the program.)

In the second presidential debate, Kerry accused Bush of "moving to the creation of our own bunker-busting nuclear weapon." Adding, "It's very hard to get other countries to give up their weapons when you're busy developing a new one."

Well, the fact is that the Bush administration is only studying the feasibility of modifying two existing nuclear weapons (the B61 and B83) into a new bomb dubbed the Robust Nuclear Earth Penetrator (RNEP).

Using a low-yield nuclear detonation, this repackaged weapon would be able to penetrate up to 30 feet into the ground to destroy reinforced underground facilities. (In the late '90s, the Clinton administration modified the B61 to penetrate up to 10 feet of frozen soil.)

Is such a weapon needed? Well, the U.S. intelligence community believes there are thousands of hard and deeply buried targets (HDBT) worldwide, many in places like North Korea and Iran.

These facilities have been built to house political leadership, military command and control centers - and missiles and labs to develop and produce WMD.

The problem is that our current conventional (i.e., high explosive) earth-penetrators can't incapacitate these facilities: We need a weapon with a little more (nuclear) punch.

The issue is rife with controversy. Some contend that developing a new nuke violates existing U.N. weapons treaties. Others worry that our having these bunker-busters increases the chances that we'll actually use a nuke.

Proponents, on the other hand, argue the new bunker-buster would 1) deter new WMD programs (why make the investment if you can't keep it safe?) and 2) give us a practical option if deterrence doesn't work.

At this point, though, the nuclear bunker-buster is just an engineering study - nothing more, nothing less. It's but one of many weapons, including new conventional weapons, that are being looked at to deal with hardened, deeply buried targets.

The fact is that evolving threats require our government to continually reassess and adjust its ability to respond effectively to new challenges. 9/11 taught us that anything less is foolhardy.

In light of the WMD-proliferation threat and the increased use of underground facilities to conceal it, looking at adding RNEPs to our arsenal only makes sense. The safety and security of our nation may depend upon it.

Peter Brookes is a Heritage Foundation senior fellow. E-mail: [email protected]