In the debate tonight, Sen. Kerry made an aside about cutting the money to develop a new, "bunker-busting" nuclear weapon. What's he talking about?

Some of the bad guys' most lethal arsenals are assumed to be buried in deep, underground caverns -- places that America's current arsenal has trouble hitting. So the Bush Administration would like to build a nuclear bomb -- a "Robust Nuclear Earth Penetrator" -- that burrows into the ground before unleashing all kinds of atomic hell. The San Jose Mercury-News explained how it might work in a story last year:

A nuclear penetrator is built in the shape of a thin cylinder with a pointed nose. Dropped from an airplane, its weight and speed allow it to smash through the surface of the ground or puncture rock or concrete. It buries itself 20 to 30 feet deep before exploding, Fred Celec, the deputy assistant to the secretary of defense for nuclear matters, said. The power of the explosion "couples'' with the earth to send shock waves down toward buried targets.

Anti-nuclear groups are spooked by the new brand of bomb, of course. "Because of its earth penetrating capability, the RNEP is considered by some in the Administration as a more "usable" nuclear weapon than existing nuclear weapons," says one.

But, according to the Merc-News, there may not be a whole lot of enthusiasm for the new nuke.

"If you can find somebody in a uniform in the Defense Department who can talk about a new need'' for nuclear bunker busters "without laughing, I'll buy him a cup of coffee,'' said Robert Peurifoy, a retired vice president of Sandia National Laboratory.

THERE'S MORE: No. He. Didn't. When asked about the biggest threat facing America, President Bush mentioned -- after nuclear proliferation -- his cockamamie missile defense system. You know, the one that can't pass its tests -- and is being deployed anyway. When will this guy let a bad idea go?