Israeli and U.S. intelligence officials have been watching each day as Iran digs a vast tunnel network just south of the Natanz nuclear production site, in what they believe is Tehran’s biggest effort yet to construct new nuclear facilities so deep in the mountains that they can withstand bunker-busting bombs and cyberattacks.

Although the construction is evident on satellite photographs and has been monitored by groups that track the proliferation of new nuclear facilities, officials from U.S. President Joe Biden's administration have never talked about it in public and Israel’s defense minister has mentioned it just once, in a single sentence in a speech last month. In interviews with national security officials in both nations, there clearly were differing interpretations of exactly how the Iranians may intend to use the site, and even how urgent a threat it poses.

But as Biden prepares for his first trip as president to the Middle East next month — one that will take him to Israel and Saudi Arabia, Iran’s two biggest regional rivals — there is little debate that the conflict over Iran’s nuclear program is about to flare again.