Had a question about underground nuclear detonation.

[Ariphaos]

As Kuroneko already mentioned, a cave-in is already a magnitude 8.8-9.0 event (depending on the actual thickness and density of the roof). Anything that collapses it is going to release 4-5 times the amount of energy that your original nukes were going to anyway. The nukes just mercifully vaporize every living thing in the tunnel system first.

You'd have to define the limits of magic in your own setting, though. Back of the envelope calculations suggest that the roof is under ~100-150 MPa of downward pressure, and if that magic works both ways, you'd want to scale your explosives back by a few orders of magnitude if you didn't want to overwhelm that.