WASHINGTON/TEHRAN — A mysterious underground complex being built beneath a massive granite mountain near Iran’s Natanz nuclear facility has seen an unusual increase in construction activity since the beginning of 2026, according to a new satellite imagery analysis. The findings come as U.S. President Donald Trump continues to warn that Washington could strike the site, adding another layer of tension to Iran’s nuclear program, U.S. military planning, and the broader conflict in the Middle East.

According to a September 9 analysis by the Washington-based Center for Strategic and International Studies (CSIS), a review of multiple types of satellite imagery covering the period from 2020 through 2026 shows that activity at Pickaxe Mountain has increased sharply this year. Researchers examined Sentinel-1 radar imagery, Sentinel-2 optical imagery, and higher-resolution commercial images from Airbus and Satellogic.

CSIS said the work now appears to have moved beyond major tunneling and excavation toward possible internal construction and reinforcement of the site’s external infrastructure. Analysts observed hardened tunnel entrances, newly paved internal roads, strengthened access areas, and the removal or leveling of soil and rock that had previously been excavated during tunnel construction.

Pickaxe Mountain, known in Persian as Kuh-e Kolang Gaz La, lies roughly two kilometers south of Iran’s Natanz uranium-enrichment complex and about 220 kilometers south of Tehran. Satellite analysis indicates that the site contains at least two deeply buried tunnel systems. Because the facility is located beneath a large granite mountain, military experts believe it would be extremely difficult to destroy with a conventional airstrike.

The history of the project goes back to 2020. After sabotage damaged an above-ground centrifuge assembly workshop at Natanz, then-Iranian nuclear chief Ali Akbar Salehi announced that a new centrifuge assembly facility would be built underground in the nearby mountains. Satellite imagery has since shown continuing tunneling and construction in the Pickaxe Mountain area.

However, the site’s true purpose has still not been independently confirmed.

CSIS identified three main possibilities. The facility could be the underground centrifuge assembly plant that Iran publicly announced. It could also be intended as a highly protected future uranium-enrichment site. A third possibility is that it is being prepared as a secure location for sensitive nuclear-related work, including uranium metal conversion, storage of equipment, or other activities relevant to a potential weapons program.

Israeli intelligence assessments have suggested that Iran may be moving centrifuges and associated equipment from damaged nuclear facilities into Pickaxe Mountain.

CSIS, however, emphasized an important limitation: satellite imagery alone cannot confirm or disprove whether centrifuges have actually been transferred there. The group also noted that the pattern of continuing construction, together with limited visible non-construction vehicle activity, may indicate that the facility is not yet operational for enrichment, assuming enrichment is its intended purpose.

For that reason, describing Pickaxe Mountain as an already functioning nuclear weapons production site would go beyond the available public evidence.

Even so, the U.S. administration appears to view the site as a serious strategic concern.

President Trump said on September 4 that the United States could strike Pickaxe Mountain “very soon,” while asserting that Washington was closely monitoring activity there. He has repeatedly warned Tehran against rebuilding or expanding sensitive parts of its nuclear infrastructure.

The confrontation is part of a much larger escalation between the United States and Iran.

The current phase of direct conflict intensified on February 28, 2026, when U.S. forces launched Operation Epic Fury. According to U.S. Central Command, more than 1,700 targets were struck inside Iran during the first 72 hours, including command centers, air-defense systems, ballistic missile infrastructure, naval facilities, and other military sites.

But the nuclear confrontation had already escalated dramatically before that.

In June 2025, the United States carried out Operation Midnight Hammer, targeting the Iranian nuclear facilities at Fordow, Natanz, and Isfahan. The U.S. military used GBU-57 Massive Ordnance Penetrator bombs, among the most powerful conventional bunker-buster weapons in the American arsenal.

U.S. officials said those strikes caused major damage to key elements of Iran’s nuclear infrastructure.

Yet very deeply buried underground facilities continue to present a major military challenge.

The roughly 30,000-pound GBU-57 is designed to penetrate hardened underground targets, but experts question whether even this weapon could reliably destroy a facility buried deeply beneath a massive granite mountain such as Pickaxe. CSIS similarly concluded that such a site may be extremely difficult to eliminate even with the most powerful non-nuclear penetrator weapons currently available to the United States.

That problem appears to be one reason the U.S. military is developing a more advanced generation of bunker-busting weapons.

The U.S. Air Force is working on a successor to the GBU-57 known as the Next Generation Penetrator, designated GBU-76/B. In June 2026, the Air Force sought additional information from defense contractors concerning development, production, and logistical support for the new weapon.

Prototype work had already begun under a contract awarded in 2025, with Applied Research Associates involved in development and Boeing supporting guidance and tail-kit systems. The goal is to create a new conventional weapon capable of attacking underground facilities that are even deeper or more heavily reinforced than those targeted by existing bunker-buster bombs.

At the same time, reporting based on U.S. federal contracting records has raised questions about military testing conducted at White Sands Missile Range in New Mexico shortly before Operation Epic Fury began.

The remote site, known as the Alternate Seismic Hardrock in Situ Test Site, is also carved into granite and has been used for testing the effects of weapons against deeply buried structures.

Federal contracting documents showed preparations for work involving underground test infrastructure, including repairs to a tunnel, installation support for test equipment, and measures intended to allow safe re-entry following a live-fire test.

Satellite imagery reportedly showed excavation and vehicle activity at the site in February and March 2026.

The timing has fueled speculation that the test preparations were connected to efforts to develop or validate capabilities against Iran’s deepest underground sites. However, there has been no public confirmation that a specific classified test was actually conducted, or that the activity was directly tied to Pickaxe Mountain.

Another major concern surrounding Pickaxe is the limited access available to the International Atomic Energy Agency, or IAEA.

The IAEA has sought additional information from Iran about the underground project, but the absence of direct and comprehensive inspection means that many outside assessments continue to rely heavily on satellite imagery and intelligence information.

CSIS said Iran has not provided sufficient responses to questions about the facility’s purpose and status.

The dispute is further complicated by Iran’s stockpile of highly enriched uranium.

Before the 2025 military strikes, the IAEA had estimated that Iran possessed about 440.9 kilograms of uranium enriched to 60 percent purity. That level is far above what is typically required for civilian nuclear power generation, though it is still below the approximately 90 percent enrichment level generally associated with weapons-grade material.

Iran has consistently denied that it is seeking a nuclear weapon and maintains that its nuclear program is intended for peaceful purposes.

That leaves Pickaxe Mountain at the center of an increasingly dangerous strategic dilemma.

For Washington, the challenge is straightforward: if the site is truly being designed as a protected sanctuary for Iran’s nuclear program, it may be extremely difficult to destroy with conventional airpower. But even if the United States were able to damage the facility, Iran could respond by shifting sensitive activities to even deeper, smaller, or more secret underground locations.

CSIS therefore warned against assuming that repeated military strikes alone can permanently solve the Iranian nuclear problem.

Its analysis suggested that military action might delay or disrupt parts of the program, but would not necessarily eliminate Iran’s technical knowledge, nuclear material, or ability to rebuild.

For that reason, the group argued that a verifiable diplomatic arrangement would ultimately be necessary if the goal is to place lasting limits on Iran’s nuclear capabilities.

The surge in construction visible over six years of satellite imagery, President Trump’s repeated military warnings, the Pentagon’s interest in next-generation bunker-buster weapons, and continuing uncertainty over IAEA access have now turned Pickaxe Mountain into one of the most sensitive flashpoints in the U.S.-Iran confrontation.

The most important question remains unanswered:

What exactly is Iran building deep beneath the granite of Pickaxe Mountain?

So far, no publicly available evidence provides a definitive answer. But the scale of construction, the apparent hardening of the underground complex, and the growing concern in Washington and Jerusalem suggest that the site is likely to remain at the center of any future escalation over Iran’s nuclear program.