Last September, when Iran’s uranium enrichment plant buried inside a mountain near the holy city of Qum was revealed, the episode cast light on a wider pattern: Over the past decade, Iran has quietly hidden an increasingly large part of its atomic complex in networks of tunnels and bunkers across the country.

Iran's nuclear plant at Isfahan has many buildings above ground, but American nuclear analysts say that Iran has also filled the nearby mountains with tunnels.

In doing so, American government and private experts say, Iran has achieved a double purpose. Not only has it shielded its infrastructure from military attack in warrens of dense rock, but it has further obscured the scale and nature of its notoriously opaque nuclear effort. The discovery of the Qum plant only heightened fears about other undeclared sites.

Now, with the passing of President Obama’s year-end deadline for diplomatic progress, that cloak of invisibility has emerged as something of a stealth weapon, complicating the West’s military and geopolitical calculus.

The Obama administration says it is hoping to take advantage of domestic political unrest and disarray in Iran’s nuclear program to press for a regimen of strong and immediate new sanctions. But a crucial factor behind that push for nonmilitary solutions, some analysts say, is Iran’s tunneling — what Tehran calls its strategy of “passive defense.”

Indeed, Secretary of Defense Robert M. Gates has repeatedly discounted the possibility of a military strike, saying that it would only slow Iran’s nuclear ambitions by one to three years while driving the program further underground.

Some analysts say that Israel, which has taken the hardest line on Iran, may be especially hampered, given its less formidable military and intelligence abilities.

“It complicates your targeting,” said Richard L. Russell, a former Central Intelligence Agency analyst now at the National Defense University. “We’re used to facilities being above ground. Underground, it becomes literally a black hole. You can’t be sure what’s taking place.”

Even the Israelis concede that solid rock can render bombs useless. Late last month, the Israeli defense minister, Ehud Barak, told Parliament that the Qum plant was “located in bunkers that cannot be destroyed through a conventional attack.”

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