On the left, a GoogleEarth image dating from March 2005 showing the uncovered facilities existing at that time (before being buried); on the right, the probable main centrifuge hall as it appeared in late September 2009, also before being buried.
At the risk of stating the obvious, there are no tunnels at the Qom site; its all cut and cover. In the March 2005 image, shown on the left above, Iran has leveled the “valleys” on top of the hill and then constructed “huts” to be buried; I have labeled those Hall 1 and Hall 2. Hall 3, shown in the ISIS/DigitalGlobe image on the right, has been constructed in an excavation. If you measure the floor areas of these three halls, 1 and 2 each have an area of 1000 square meters while Hall 3 has an area of 4180 square meters (roughly!). Halls 1 and 2 have been covered in the ISIS/DigitalGlobe image with soil (previously stored somewhere not yet identified) while Hall 3 still remains unburied.
Using the average area per effective centrifuge I calculated for a previous post, Hall 3 can hold 2900 centrifuges. The <a href=http://www.armscontrolwonk.com/2478/how-much-dirt < earthen berm whose volume I calculated before nicely matches the hole for Hall 3. If Halls 1 and 2 are also used for centrifuges, the entire site could contain 4300 centrifuges. It is interesting to note that if Hall three is covered with only the soil removed from the hole, it will only be buried to a depth of less than 5 meters. Since Hall 3 has not been covered, it seems safe to assume that they have not installed any centrifuges in it yet. Installing the supporting infrastructure—air conditioning, power and control cables—is undoubtedly facilitated by working on the unburied site.