The site looms up from the Iranian desert like something out of a thriller novel, ringed by antiaircraft batteries, a security perimeter two miles around, and huge tunnels that lead deep into the mountainous complex.

Welcome to Fordo. Once a covert site of Iran’s sprawling nuclear program, then a closely monitored uranium enrichment plant, it is now a flash point in this week’s preliminary nuclear deal between the West and Tehran. The question is whether the proposed conversion of the site to a peaceful research center will prove effective or instead produce an illusion that eventually aids Iran’s pursuit of an atom bomb.

Fordo — some 20 miles from the holy city of Qum, deep inside an Iranian Revolutionary Guards base — came to public attention in 2009 when President Obama announced its existence.

The secret plant, Mr. Obama said, “represents a direct challenge to the basic foundation of the nonproliferation regime.” The Iranians, he added, “are going to have to come clean.”