Call it Plan B for dealing with Iran, which recently revealed a long-suspected nuclear site deep inside a mountain near the holy city of Qom.
The 15-ton behemoth _ called the "massive ordnance penetrator," or MOP _ will be the largest non-nuclear bomb in the U.S. arsenal and will carry 5,300 pounds of explosives. The bomb is about 10 times more powerful than the weapon it is designed to replace.
The Pentagon has awarded a nearly $52 million contract to speed up placement of the bomb aboard the B-2 Stealth bomber, and officials say the bomb could be fielded as soon as next summer.
Pentagon officials acknowledge that the new bomb is intended to blow up fortified sites like those used by Iran and North Korea for their nuclear programs, but they deny there is a specific target in mind.
"I don't think anybody can divine potential targets," Pentagon press secretary Geoff Morrell said. "This is just a capability that we think is necessary given the world we live in."
The Obama administration has struggled to counter suspicions lingering from George W. Bush's presidency that the United States is either planning to bomb Iranian nuclear facilities itself or would look the other way if Israel did the same.
The administration has been careful not to take military action off the table even as it reaches out to Iran with historic talks this month. Tougher sanctions are the immediate backup if diplomacy fails to stop what the West fears is a drive for a nuclear weapon.
Defense Secretary Robert Gates recently said a strike against Iran's nuclear facilities would probably only buy time. Joint Chiefs of Staff Chairman Adm. Mike Mullen has called a strike an option he doesn't want to use.
The new U.S. bomb would be the culmination of planning begun in the Bush years. The Obama administration's plans to bring the bomb on line more quickly indicate that the weapon is still part of the long-range backup plan.
"Without going into any intelligence, there are countries that have used technology to go further underground and to take those facilities and make them hardened," Pentagon spokesman Bryan Whitman said. "This is not a new phenomenon, but it is a growing one."
After testing began in 2007, development of the bomb was slowed by about two years because of budgetary issues, Whitman said, and the administration moved last summer to return to the previous schedule.
North Korea, led by Kim Jong Il, is a known nuclear weapons state and has exploded working devices underground. The United States and other countries have offered to buy out the country's weapons program. The Obama administration is trying to lure Pyongyang back to the bargaining table after a walkout last year.
Iran is a more complex case, for both diplomatic and technical reasons. Iran's president, Mahmoud Ahmadinejad, claims its nuclear program is peaceful and meant only to produce energy, but the West suspects a covert bomb program that may be only a year or so away from fruition.
"I don't really see it as a near-term indication of anything being planned. I think certainly down the road it has a certain deterrent factor," said Kenneth Katzman, a specialist on Iran and the Middle East at the Congressional Research Service. "It adds to the calculus, let's say, of Ahmadinejad and Kim Jong Il."
Details about Iran's once-secret program have come out slowly and often under duress, as with last month's surprise confirmation of the hidden underground development site near Qom.
That revelation came a month after the Pentagon had asked Congress to shift money to speed up the MOP program, although U.S. and other intelligence agencies had suspected for years that Iran was still hiding at least one nuclear development ite.
The MOP could, in theory, take out bunkers such as those Saddam Hussein had begun to construct for weapons programs in Iraq, or flatten the kind of cave and tunnel networks that allowed al-Qaida leader Osama bin Laden to escape U.S. assault in Tora Bora, Afghanistan, shortly after the U.S. invasion in 2001.
The precision-guided bomb is designed to drill through earth and almost any underground encasement to reach weapons depots, labs or hideouts.