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.

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.”