The New York Times reports Wednesday that Iran has been hiding much of its nuclear infrastructure in tunnels and bunkers across the country over the past decade.
The Times says that by doing so Iran has protected its nuclear infrastructure from attack, and has also obscured the scale and nature of its atomic program.
The paper cites unnamed U.S. government and private experts as its sources for the article.
It says its sources report there may be thousands of big tunnels in heavily mountainous Iran, but that the purpose of all of the tunnels is not clear.
Iran has acknowledged it is developing uranium enrichment facilities at several sites, including Isfahan and Natanz. But Iran says all of its nuclear activities are for peaceful purposes - mainly to generate electricity.
In September, U.S. President Barack Obama and other Western leaders accused Iran of building a secret nuclear plant inside a mountain in the city of Qum, south of Tehran. Iran admitted developing the facility but denied it had been kept a secret.
The Times reports that Iranian President Mahmoud Ahmadinejad has long played a prominent role in developing Iran's tunnel infrastructure, first as an engineer and founder of Iran's Tunneling Association, and now as president.