The United States may possess only a few of its largest bunker-busting bombs left, according to a new analysis from the Center for Strategic and International Studies. The assessment, released on Thursday, comes as President Donald Trump weighs potential strikes on Iran using the GBU-43/B Massive Ordnance Air Blast bomb, commonly called the "mother of all bombs."

The CSIS review of U.S. ordnance inventories warns that adversaries are constructing deeper underground facilities to evade existing weapons, some of which are now in increasingly short supply. Report authors Mark Cancian and Chris Park noted that "these military facilities will be both more numerous and harder to reach than in the recent past."

The United States maintains a range of bunker-busting munitions, including 2,000-pound and 5,000-pound weapons that have seen extensive use in operations against Iran. However, inventory concerns are most acute for the largest systems. The GBU-57 Massive Ordnance Penetrator, a 30,000-pound bomb designed to destroy deeply buried targets, is among those facing shortages. The Air Force deployed 14 GBU-57s against Iranian nuclear sites during Operation Midnight Hammer in June 2025. Although the Pentagon has allocated $123 million toward replenishing its stockpile, new production will take several years to materialize, the report states.

Publicly available procurement data cited by analysts suggests the United States now holds only a low-single-digit number of GBU-57 bombs. The Pentagon has not disclosed its current inventory figures.

The GBU-57 can penetrate up to 200 feet of earth and is delivered by the Air Force’s B-2 stealth bomber. Iran has reportedly expanded underground infrastructure, including 33 missile bases near the Strait of Hormuz that may have survived earlier American attacks. Another site of concern is the Pickaxe Mountain nuclear complex near Natanz, which analysts believe may be buried so deeply within granite that even the GBU-57 could struggle to reach it. Construction continues at the facility, and its operational status remains unclear.

The analysis identifies two primary threats: the need to strike moderately difficult tunnel targets requiring large volumes of munitions, and the challenge of penetrating deeply buried mountain facilities. U.S. Central Command’s Admiral Brad Cooper has urged greater investment in weapons capable of engaging hardened underground sites, stating, "Everybody is going underground."

The Pentagon has explored options involving the GBU-43/B bomb, which weighs approximately 21,000 pounds and was previously used in 2017 to target an Islamic State cave complex in Afghanistan. While larger than some conventional weapons, the GBU-43/B cannot penetrate as deeply as the GBU-57, according to the CSIS report. The think tank concluded that existing U.S. weapons may be insufficient to reach the most hardened and deeply buried Iranian facilities.