Journalists have been allowed to accompany Iranian President Mohammad Khatami on his first official visit to the Natanz nuclear facility.
Iran insists Natanz is helping to develop nuclear energy, not bombs
Natanz, some 250km (150 miles) south of Tehran, was a closely-guarded secret until late 2002 when its existence was revealed by an Iranian exile group.
The site holds uranium enrichment facilities which, the US says, could be aimed at producing nuclear weapons.
But Tehran insists its nuclear programme is for peaceful purposes.
In an unusual gesture, Tehran allowed more than 30 local and foreign journalists to accompany President Khatami on a tour of Natanz.
They were taken deep inside a building to a vast empty hall designed to house 50,000 enrichment centrifuges, a Reuters news agency report said.
The enrichment facility was built more than 18m (54ft) below ground because of "security problems", Iranian officials were quoted as saying.
But defence experts say it could be a precaution against possible aerial attacks from the US and Israel.
Dozens of anti-aircraft placements were also spotted by journalists on the approach to Natanz.
Anti-aircraft gun placements could be seen around Natanz
Mohammad Saeedi, deputy head of Iran's Atomic Energy Organisation, said the site was being inspected by the UN's International Atomic Energy Agency.
"IAEA inspectors visit this facility at least once a month and also use a monitoring system to check the suspension," he said.
But he said staff were keen to resume enriching uranium, which was frozen while Iran negotiates with the EU over its nuclear programme.
"The people involved in the project are frustrated by the suspension," he said. "They hope there is an agreement with the Europeans so that all activities can be resumed. Enrichment is Iran's right."