The fun part about this, to me, is that if this pans out, it will be impossible for anyone to plausibly say that we 'planted' the evidence.
You can't 'plant' a building underground in an area with such a high water table. How long the building was there will be easily discovered.
They do tend to want to "pop up" out of the ground... I can see the headlines now...
Nuclear Ractor Surfaces in Iraq
Surprise!