Geological testing was done at the site in 2005, for the purpose of placing pillars in solid ground so that the stability of the roof would not be an issue in the event of an earthquake.
What they found while using high resolution travel time tomography, a method of getting images from under the surface of the earth using waves of energy, were underground cavities. These were both man made and natural. The man made gaps in the earth were filled with rocks, ceramics, and other items of interest to archeologists.
Before drilling the new shafts and setting the pillars that would support the new roof, the geologists relied on the images to choose the most stable locations.
Source: Examiner