President Bush signed a huge new defense bill that includes millions of dollars for a small nuclear bomb designed to destroy deep, hardened underground bunkers.

Among the many items tucked away in the $401 billion defense authorization act was a $15 million three-year research project by the Energy and Defense departments to create the Robust Nuclear Earth Penetrator.

--New York Post

So it's official -- we've crossed that line; we're researching a new, "thinkable" battlefield nuclear weapon. Several times since I started this blog I've linked this Popular Mechanics story about the proposed weapons. It's worth reading:

Rob Nelson, a physicist with the Princeton University Program on Science and Global Security, and an expert on nuclear weapons design, has looked carefully at the relationship between the depth of a primary-powered explosion and geological damage. He argues that the sort of deep penetrator proposed by Younger would, in fact, release rather than contain radioactive fallout. While it is true that most material would remain within the blast area, a radioactive cloud seeping from the crater would release a plume of gases that would irradiate anyone in its path.

He has calculated that a weapon with a yield of about 0.1 kiloton--about one two-hundredth the energy of the bomb dropped on Hiroshima--would have to penetrate to a depth of 230 ft. to fully contain the explosion in the manner that Younger has described. Nelson cautions that if it were used to root out terrorists near a major Third World city such as Baghdad, the casualties could be in the hundreds of thousands....