The GBU-57 series MOP (Guided Bomb Unit / Massive Ordnance Penetrator) is a 30,000-pound (14,000 kg) class, 20.5-foot-long (6.2 m) precision-guided munition "bunker buster" bomb developed by Boeing for the United States Air Force (USAF).[6][4] Composed of a BLU-127 bomb body and an integrated GPS/INS guidance package, there are seven GBU-57 variants, the most recent being the GBU-57F/B.[4] Due to its size and weight, the GBU-57 MOP can only be carried by the Northrop B-2 Spirit strategic bomber[7][8] and the B-21 Raider,[9] although initial tests were conducted with a modified Boeing B-52 Stratofortress.[10][11] The first combat use of the GBU-57 MOP came on June 22, 2025, when seven Northrop B-2 Spirit stealth bombers dropped 14 GBU-57 bombs on Iran's Fordow Uranium Enrichment Plant and Natanz Nuclear Facility. The bomb is much larger than earlier USAF bunker-busters such as the 5,000-pound (2,300 kg) GBU-28 and GBU-37.[12]