A bunker-penetrating warhead is largely a myth, a misperception on how earth-penetrating weapons actually work.
Contrary to popular conception, earth-penetrating weapons work by coupling their explosive power with the ground, not by digging down to the bunker and exploding inside it. By simply penetrating as little as two to three meters into the ground, coupling can be achieved. The explosion of the warhead then generates a destructive shockwave, much like the effects of an earthquake, that can damage or destroy anything within a quantifiable range of the explosion.
Here is an animation that explains visually how a nuclear earth-penetrating weapon works.