Near a peak of the Zagros Mountains in central Iran, workers are building a nuclear facility so deep in the earth that it is likely beyond the range of a last-ditch U.S. weapon designed to destroy such sites, according to experts and satellite imagery analyzed by The Associated Press.

The photos and videos from Planet Labs PBC show Iran has been digging tunnels in the mountain near the Natanz nuclear site, which has come under repeated sabotage attacks amid Tehran’s standoff with the West over its atomic program.

With Iran now producing uranium close to weapons-grade levels after the collapse of its nuclear deal with world powers, the installation complicates the West’s efforts to halt Tehran from potentially developing an atomic bomb as diplomacy over its nuclear program remains stalled.

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Completion of such a facility “would be a nightmare scenario that risks igniting a new escalatory spiral,” warned Kelsey Davenport, the director of nonproliferation policy at the Washington-based Arms Control Association. “Given how close Iran is to a bomb, it has very little room to ratchet up its program without tripping U.S. and Israeli red lines. So at this point, any further escalation increases the risk of conflict.”

Read more: AP