OOC: Bombing a large underground facility from the inside would be like trying to kill a person by injecting a large amount of saline solution into them (saline solution is salt water, extremely nonlethal). Yes, it's going to cause a noticeable amount of damage, but unless you do it somewhere you can damage something extremely vital in just the right way, you're not going to kill them.
Not to mention, it would be a bitch to get anything, magic, EM waves (radio, X-ray, gamma), or whatever, through a mountain to signal the explosion that would hit that vital thing. Any sort of device to extend it is liable to be detected, and directly sending the signal through the mountain opens the possibility that the signal is blocked by the mountain, detected and blocked by whoever is inside, or is blocked by something the people inside have installed in the mountain.
The bombs would be best used as a distraction.