Actually, it is the Muslims that have been digging under the Mount in an area called Solomon's Stables - to build a huge underground mosque there. That is what is causing a lot of unstability. Many say that it may well cause a collapse of the whole structure and may be the reason for a bulge on the outer wall.
Several years ago, Israel did dig out a tunnel on the OUTSIDE of the retaining wall and found some pretty cool stuff - including a gate into the Temple Mount area.
Yes I remember the gate part of the story. but are they sure the Muslims are building an underground Mosque?