The facility, buried in a mountain outside the city of Qom, is still under construction (satellite photo above). It is designed to enrich uranium and could be used to produce the essential material for a nuclear weapon.
On Friday the International Atomic Energy Agency, said that Tehran wrote to the agency on September 21, disclosing that it is building a new uranium enrichment facility.
The revelation has heightened fears that Iran is determined to develop the capacity to build an atomic weapon, although Tehran insists that it wants a nuclear programme to generate energy for a rapidly growing population.
The programme presents the West with a thorny challenge. World leaders meeting this week in the United States have been considering a response. Their options include diplomatic pressure, tougher sanctions, and in the last resort military action. One scenario is that Israel, which feels threatened by Iran, could bomb the plants.
Israel said on Saturday that this week's disclosure proved the country was seeking nuclear weapons, and demanded an "unequivocal" Western response.