Flood the tunnels, use drones to place underwater explosives. Upon detonation the over pressure wave would cause numerous cave ins and damage to any remaining infrastructure.
Read my post below. After pumping in the gas, clear GG the area and blow it all.
That’s kind of what I was thinking. Combine flooding with using high explosives, so that the water would transmit the shockwave of an explosion.
They could send little GPS floating Yellow Rubber Ducks in with the seawater. Then when the ducks gets to their underground warehouse/bunkers they will have the exact GPS location and depth to drop the bunker buster bomb.
Quack Quack