The San Jose Mercury News is reporting a "significant shift in America's nuclear strategy": "The Bush administration intends to produce -- not just research -- a thermonuclear bunker-busting bomb to destroy hardened, deeply buried targets, the Pentagon has acknowledged for the first time."

Yesterday, I reported on the opening phases of a new U.S. effort to build up its nuclear arsenal. Here's where that effort is heading:

The weapon -- known as the Robust Nuclear Earth Penetrator -- would be a full-power hydrogen bomb that would throw up enormous clouds of radioactive dust while wreaking large-scale damage and death if used in an urban area. It would be thousands of times more powerful than the conventional "bunker busters'' dropped on Baghdad in an attempt to kill former Iraqi leader Saddam Hussein.

Federal officials signed documents in Washington this week to launch a preliminary design contest between Lawrence Livermore National Laboratory and Los Alamos National Laboratory in New Mexico...

Fred Celec, the deputy assistant to the secretary of defense for nuclear matters, made clear that the administration wants the weapon and is moving forward.

If a hydrogen bomb can be successfully designed to survive a crash through hard rock or concrete and still explode, "It will ultimately get fielded,'' Celec said in an interview with the Mercury News. The United States has worked on nuclear earth penetrators for decades, and scientists involved in the project say they expect to succeed.

The project is integral to the administration's push to move away from Cold War "city scraping'' ballistic missile warheads to battlefield "tactical'' weapons.