Steve Blank

Deeply Buried Targets Versus Bunker Busters. One of the stated goals of the U.S. strikes on Iran was to remove their capacity to build nuclear weapons. This required the U.S. to destroy their deeply buried underground nuclear enrichment sites. The largest conventional bomb in the U.S. military’s arsenal is the 30,000-pound GBU-57 Massive Ordnance Penetrator, carried by the B-2 Bomber. A 2019 description said it could reach targets as far as 200 feet underground (its actual effectiveness depends on geology, concrete, facility design, entry points, and the use of multiple weapons). The Iranians knew this and built their nuclear enrichment sites underground. The Fordow site is ~260 to 300 feet under limestone and dolomite. The Natanz site is 25 to 160 feet underground, protected by a 25-foot concrete shield. (But public estimates are all over the place.)

Iran declared Isfahan as an enrichment complex to the IAEA the day before the June 2025 strikes; inspectors never got inside it, therefore its depth and geology are unknown, but speculation is that the enrichment complex is 260 to 330 feet underground. Another suspected enrichment facility is in Pickaxe Mountain, where the exact depths, geology, are unknown but estimates put the facility 250 – 330 feet below the summit. Both Isfahan and Pickaxe were Iran’s shift toward “deep mountain basing” to escape the reach of conventional military strikes.

In June 2025 in Operation Midnight Hammer U.S. B-2 bombers dropped 14 GBU-57 bombs on top of Fordow and Natanz. At Isfahan (Esfahan) more than two-dozen submarine-launched Tomahawks hit tunnel entrances and surface targets. In March 2026, strikes at Natanz hit its entrance. 

According to the Institute for Science and International Security, the 22,000 centrifuges at Natanz, the Natanz pilot plant, and Fordow are almost certainly wrecked. What may have survived is the enriched uranium, the spare parts, and the people, and that is the problem. The International Atomic Energy Agency estimated (in June 2025) that Iran had about 970 pounds of uranium enriched to 60%; it has since lost track of where it is. Iran’s known uranium conversion capability at Isfahan was damaged, which it needed to turn the stockpile of enriched uranium into a weapon, and there is no public evidence that enrichment has resumed at Natanz, Fordow or Isfahan. We never hit Pickaxe Mountain or the Isfahan enrichment facility with the GBU-57. The strikes have set Iran back, but no one knows for how long. 

While the GBU-57 MOP cannot cleanly punch through solid rock, the debate has shifted from an absolute physical destruction calculation to a functional denial calculation—meaning the military’s goal would be to seal Iran’s nuclear assets in a mountain tomb rather than blowing them up directly.

Did We Succeed? Our stated goal was to prevent Iran from rapidly rebuilding the capability to build nuclear weapons, but there’s no certainty that we did that. Repeated airstrikes may have damaged the buried enrichment plants, but we can’t prove it or account for the buried enriched uranium inside. The fact is that it’s extremely difficult to see how much damage the bombs did to Hard and Deeply Buried Targets (HDBTs) from the air, and without inspectors on the ground, knowing for sure is guesswork. Putting boots on the ground would carry enormous costs and risks. Negotiating the removal and inspection of weapons grade uranium would depend on Iranian cooperation and a way to verify we got it all. All bad choices.

There is another emerging option: autonomous drilling equipment.

Subterranean Sensing and Defeat. It’s not hard to imagine that with enough boots on the ground the U.S. could secure a landing zone, fly in drilling equipment and have crews drill hundreds of feet vertically or thousands horizontally to either place sensors and/or explosives. The U.S. has had organizations building specialized equipment and training for breaching tunnels in Combatant Commands and scattered across the DoW for years (DTRA, DARPA, Army ERDC and CAVES and CZ5 programs, SOCOM, Sandia and the Army’s half-billion-dollar underground-training push (2018) and ATP 3-21.51 Subterranean Operations.) One can even imagine that there are teams already trained and equipment ready to go. And one can equally imagine the Iranians are waiting for them.

What’s new is autonomous drills that could provide post-strike bomb damage assessments and/or destroy Hard and Deeply Buried Targets (HDBTs) without seizing the site. They might be inserted covertly; more likely they’d need air cover while they work.

The combination of AI, robotics and autonomy moves this from science fiction to the possible.

Anduril, The Boring Company, Exodigo, Phoenix Boring, Earthgrid and Traysar are possible candidates to deliver these systems. There are likely others as well.

(Disclosure: after my first blog about undergrounding critical assets, I invested and have a financial interest in companies in this space.)

Surface survivability and subterranean sensing and defeat are no longer one Service’s problem, and they won’t be solved by a single system or an incremental fix. Something fundamentally disruptive has changed in the nature of asymmetric warfare and there’s no going back. While we’re actively chasing immediate solutions (Golden Dome, JIATF-401, et al), we need to rethink the nature of force protection, and military and civilian infrastructure protection. 

The U.S. needs a coherent protection and survivability strategy across the DoW and all sectors of our economy. This conversation needs to be not only about how we do it, but how we organize it, how we budget and pay for it, and how we rapidly deploy it.

Lessons Learned

• The DoW needs to organize its scattered subterranean assets into a single organization: a Direct Reporting Portfolio Manager (DRPM) for Subterranean Warfare, reporting to the Deputy Secretary of War
  • It should have three portfolios – Resilience, Sensing and Defeat
  • Providing validated requirements, experimentation, testing, acquisition, and fielding  to Combatant Commands.
• Protection and survivability solutions are not as sexy as buying aircraft or weapons systems but they may be the key to winning a war.
• Subterranean Sensing and Defeat need to move from bespoke special-operations capabilities into standard doctrine.