The GBU-57A-B Massive Ordnance Penetrator (MOP) is 5,300 pounds of high explosive wrapped in 30,000 pounds of steel, and designed to obliterate fortified positions and underground bunkers from the inside.
Developed by the US Air Force with help from the Defense Threat Reduction Agency (DTRA) and built by Boeing, the $400 million MOP project is the largest Bunker Buster bomb in the US arsenal by a good 25,000 pounds; capable of burrowing through 60 feet of reinforced concrete. But the MOP may have met its match in Iran's Fordow nuclear enrichment complex. This 300-centrifuge facility isn't just tucked safely underground, it's buried under an entire mountain. Not even an MOP can punch through that much Earth—yet.
For the rest of the story: http://gizmodo.com/america-gussies-up-its-biggest-bunker-buster-to-nix-ira-492256766