“Bunker busters” is the nickname given to a class of weapons designed to attack what the Department of Defense (DOD) calls “hard and deeply buried targets” (HDBTs). These targets are typically an adversary’s most valuable assets. Although such targets have historically been relatively few, their numbers are increasing as countries and nonstate actors seek protection against U.S. precision weaponry. The United States has developed a range of weapons to attack these targets, but their large numbers and increasing hardness will require a greater U.S. effort.
The U.S. bunker buster inventory today contains three types of weapons to cover different situations. All, except the B83-1 and B61-11, can use precision guidance to hit their target. The appendix has details on each of these weapons.
- Bombs: Most currently available bunker busters are air-dropped bombs (bomb live units, or BLUs) that can be fitted with laser or GPS guidance kits to be configured as guided bomb units (GBUs). Compared with regular bombs, bunker busters have a much heavier casing and smaller explosive charges.
- Standoff munitions: Variants of existing systems with enhanced penetrating capability were made during the Iraq and Afghanistan wars. Only the Navy’s system (JSOW-C) remains in the inventory today.
- Nuclear weapons: Because there are targets that even specially designed conventional munitions cannot destroy, nuclear weapons are also an option.
Weapons to destroy hardened targets have a long history. During World War I, the German Army had “Big Bertha,” a 17 in howitzer firing 2,000 lb projectiles and designed to pry open Belgian forts. During World War II, the British developed the Grand Slam bomb to strike underground structures and German U-boat pens in France.
Militaries with nuclear weapons built underground facilities during the Cold War to conceal and protect their operations from a nuclear attack. A major change came with the advent of precision-guided conventional munitions and their subsequent widespread adoption in U.S. warfighting, as shown in Table 2. This made many more targets vulnerable, spurring the rapid growth of HDBTs as countries and nonstate actors sought to protect key military assets. The People’s Liberation Army Rocket Force, for example, built networks of tunnels—referred to as the “Underground Great Wall”—to protect its, until recently, limited arsenal of nuclear weapons and ensure second strike capability against other nuclear powers. Many of North Korea’s nuclear and missile facilities were built underground. Al Qaeda, the Islamic State, and other adversaries during the counterterrorism campaign also took to hiding in underground bunkers or caves.
As the Trump administration considers its next steps in the Iran campaign, many hardened military targets remain. Continuing Iranian missile launches appear to confirm reports that some underground missile and drone storage facilities have survived, particularly in the reported 33 underground missile bases adjacent to the Strait of Hormuz.
The United States has used large numbers of 2,000 lb and 5,000 lb bunker busters to attack Iran’s extensive underground nuclear facilities and other protected military infrastructure. Israel, too, has used large numbers of these U.S.-made munitions in its wars since Hamas’s October 7 attack. Hamas’s estimated 310 miles of tunnels—about six times the underground length of the Washington Metro—enabled its surprise attack on October 7, 2023, and helped the group survive Israeli air and ground attacks. Nevertheless, the United States retains a large inventory of 2,000 lb bunker buster munitions. Inventory stress may occur with the largest munitions (5,000 lb), which would be needed for the surviving and likely most hardened targets.
Further, existing U.S. weapons may be insufficient to reach the most hardened and deeply buried facilities. Among Iranian nuclear sites, analysts have identified Pickaxe Mountain to be even deeper underground and likely beyond the reach of even the most powerful conventional munitions, compared to other facilities at Isfahan and Natanz. Construction at Pickaxe Mountain is ongoing, so the facility may not be ready. It is nevertheless a part of Iran’s nuclear program, and eliminating it would be necessary to fully meet President Trump’s goal of preventing the regime from ever having a nuclear weapon.
Out of the Shadows, Into the Mainstream
What were once niche weapons for use against a limited set of targets are now in great demand. The proposed FY 2027 defense budget reflects the demand for weapons against HDBTs, requesting $429 million for conventional bunker buster weapons—up from an average of $123 million (in FY 2027 dollars) in the previous 10 fiscal years. The previous spike in FY 2019 reflects the high demand to defeat the Islamic State.
A limitation is that current bunker busters are nearly all bombs because of the weight needed for heavy casings and explosive power. Delivery aircraft must fly over the target, requiring at least local air superiority. That has been feasible in recent conflicts, but it will not always be achievable, for example, in a potential conflict with China. A standoff munition would be highly desirable to reduce risk to the aircraft and expand the number of targets that could be reached.
The demand for weapons against HDBTs will continue to grow because, as colleague Eliot Cohen put it, there has been a “revolution in tunneling affairs.” U.S. adversaries are digging deeper and more extensively to protect themselves from U.S. precision weapons. These military facilities will be both more numerous and harder to reach than in the recent past. Thus, the United States needs bunker busters that can address two target sets. One is the large number of tunnels and other moderately difficult targets, which require volume. The other is a much smaller but more difficult target set of facilities inside mountains. Here, numbers matter less; the key capability is depth of penetration.
Appendix: Types of Bunker Busters
This appendix describes each of the munitions listed in Table 1, dividing them into three categories: bombs, standoff munitions, and nuclear weapons. It provides a broad view of U.S. capabilities and limitations.
Bombs
Bomb live units (BLUs) are free-fall ordnance that can be fitted with guidance kits to become Guided bomb units (GBUs). They are listed here by size and date of production. It is important to note the size differences. The GBU-43B, “Mother of all Bombs,” is 60 times the size of the BLU-140.
- BLU-140: This 500 lb bomb retains the shape of the Mk 82 500 lb general purpose bomb but has an external casing designed for penetration. The bomb’s relatively small size allows more to be loaded on aircraft, and thus more targets can be struck per sortie. It can be fitted with the Joint Direct Attack Munition (JDAM) guidance system.
- BLU-109: This 2,000 lb bomb can be precisely delivered when fitted with the Paveway laser-guided system or JDAM and reportedly can break through six feet of reinforced concrete. It . Compared with the standard 2,000 lb bomb used by U.S. forces, the Mk 84, it has a thicker steel case and a smaller explosive charge (approximately 500 lb versus 945 lb). The bomb has been widely used since its introduction in the 1980s. Recent operational uses include the Israeli Air Force strike against Hassan Nasrallah’s compound in Lebanon and U.S. operations during Operation Epic Fury. Nevertheless, a sizable inventory remains due to large procurement over decades.
- BLU-116/BLU-121: During the counterterrorism campaigns, the Defense Threat Reduction Agency procured variants of 2,000 lb bombs. BLU-116 had more than twice the penetration capability of the BLU-109. The Navy received 580 of them in 2005, but likely none remain in the inventory today. Also procured, though in limited numbers, was the BLU-121. It had a thermobaric warhead that could penetrate Taliban tunnels, not just collapse the entrances.
- BLU-137: Also referred to as the Advanced 2,000 lb Penetrator, or A2K, this is the latest variant of the 2,000 lb hard target penetrator bomb. It has “increased reliability and survivability” over the BLU-109 as the United States faced “increasingly hardened targets.” Procurement began in FY 2018.
- BLU-113: This 5,000 lb bomb has a 650 lb explosive package and the ability to penetrate over 20 feet of concrete or 100 feet of earth. It was fitted with the Paveway laser-guided system (GBU-27), with later variants adding GPS/INS guidance (GBU-28). In planning for an air campaign against Iraq following the invasion of Kuwait, deeply buried Iraqi command bunkers raised questions of whether the existing BLU-109s would be sufficient. The Air Force crash developed the GBU-28 ahead of Operation Desert Storm—the bomb body, for example, was made from an Army artillery barrel. Only two were used during the First Gulf War due to the limited inventory and rapid conclusion of the war. The BLU-122 improved the bomb’s performance.
- BLU-138: Also known as Advanced 5,000 lb Penetrator, or A5K, this bomb has been procured since FY 2023. Compatible with the JDAM guidance kit, it will replace the BLU-113/122 bombs.
- GBU-57/BLU-127: Also known as the Massive Ordnance Penetrator (MOP), this is a 30,000 lb bomb with a 5,700 lb explosive charge. The munition reportedly can penetrate up to 200 ft of unspecified material. Budget documents record a procurement quantity of only 8, but public reports indicate the Air Force took delivery of at least 20. MOPs have had a series of upgrades, driven in part by concerns over their effectiveness against increasingly deep and fortified targets. 14 MOPs were used in the attacks on Iran’s nuclear facilities at Fordow and Natanz during Operation Midnight Hammer. The Trump administration subsequently reprogrammed $123 million to replenish the inventory, but it will take several years for the bombs to be delivered. The number of MOPs remaining in the U.S. inventory is estimated to be in the low single digits.
- GBU-43/B: This 21,000 lb munition bears the moniker “The Mother of all Bombs” (MOAB). The picture below shows its massive size. The bomb casing is not designed to penetrate; rather, it uses a fuel and air explosive to reach into caves or hardened bunkers. When the bomb lands, the gaseous fuel disperses, seeping through cracks and openings to infiltrate confined spaces. A spark then ignites the mixture, resulting in a massive fireball with immense overpressure that destroys targets from the inside. Because of the large effects and potential for collateral damage, the MOAB has been used only once: in 2017, when President Trump ordered a strike against a Taliban cave complex in Afghanistan. The United States built about a dozen of these huge weapons. With one used operationally and few in testing, around 10 should remain.
Standoff Munitions
During the wars in Iraq and Afghanistan, each military service developed bunker busters with standoff range by modifying existing missiles to carry penetrating warheads. The Air Force’s Conventional Air-Launched Cruise Missile (CALCM) with a BLU-116–based warhead retired with other CALCMs in 2019. An Army Tactical Missile System (ATACMS) missile with a Navy reentry and penetrator system was canceled during development.
The Joint Standoff Weapon version C (JSOW-C) is the only standoff munition still in the U.S. inventory designed for use against hardened targets. It has a 500 lb British-developed penetrating warhead with two charges: an initial shaped charge to cut a passage and a following warhead to explode inside the target. It is not a missile, but rather glides, unpowered, 12–70 miles to the target. Range depends on the release altitude—more altitude provides a longer range. The inventory goal was initially set at 7,000. In FY 2015, the Navy reported a low inventory of the missile but ended procurement the following year based on “ongoing investment in related weapons.” Several hundred are estimated to remain in U.S. inventory.
Nuclear Weapons
Some targets are too well fortified or deeply buried for conventional munitions to destroy. The United States developed “earth-penetrating weapons”—nuclear variants of bunker buster munitions—for particularly difficult HDBTs. Nuclear blasts generate large ground shock and overpressure effects that can more readily defeat adversary hardening than conventional munitions. The United States has three nuclear bombs, all air-delivered, available for this purpose:
B83-1 is an unguided nuclear bomb developed as a nuclear bunker buster. With a yield of 1.2 MT, it is the most powerful nuclear warhead in the current U.S. stockpile. Hundreds were manufactured, but only a handful remain. The Biden administration planned to retire the B83-1 as the B61-13 was deployed. The Trump administration has not yet stated its policy.
B61 is a family of air-dropped gravity bombs that has become the main element of the U.S. tactical nuclear stockpile. One variant, the B61-11, has earth-penetrating capability but is unguided. A later variant, the B61-13, was designed against “large area and some harder targets” with precision guidance, a hardened nose cone, and steel casing. Air Force budget materials suggest a new nuclear bunker buster may be in development.
Mark F. Cancian (Colonel, U.S. Marine Corps Reserve, ret.) is a senior adviser with the Defense and Security Department at the Center for Strategic and International Studies (CSIS) in Washington, D.C. Chris H. Park is a research associate for the Arleigh A. Burke Chair in Strategy at CSIS.