Iran and North Korea bury their nuclear weapons programs deep in the earth to avoid attack. The latest answer from the Air Force: the MOP, or Massive Ordnance Penetrator—a “bunker buster” far more powerful than its predecessors. At 30,000 pounds, the satellite-guided bomb, made by Boeing, is so heavy only huge B-52 and B-2 bombers can carry it. Dropped from high altitude, it smashes through concrete reinforcements before exploding, destroying fortified underground facilities. A guide to the Pentagon’s subterranean arsenal.