[jim_trent]

I would think that an air-burst weapon would not be ideal for destroying a tunnel complex. The British in WWII had some bombs that weighed about the same, but the case was made of heavy-wall, high-strength steel. It was meant to plunge through concrete and rock, deep into the earth, and explode afterward. It was called the “earthquake bomb” and was successful against heavily armored, buried bunkers.

[RayChuang88]

Oh, you mean this?

This is the Grand Slam bomb, a bomb designed to penetrate a couple of hundred feet below the surface before the bomb exploded. Though its accuracy when used in March 1945 was not the best, its "earthquake effect" from the below-ground explosion was strong enough to bring down nearby structures.

The GBU-43/B MOAB is designed as a large-area surface blast weapon, and would not be used against the North Korean bunkers. Those would require something like the GBU-57A/B MOP bomb, which can penetrate several hundred feet below ground before the bomb detonates.