Which makes sense--particularly with respect to suitcase nukes (which have yields of about .5 kiloton, IIRC). But what about a big nuclear device of, say, 1 megaton or larger? The most powerful nuclear device ever detonated, the Tsar Bomba, was close to 50 megatons but was based on a design yield of 100 megatons.
Drop a megaton-range device with a hardened casing from altitude so that it burrows deep into a natural fissure in the park, and detonate it. Might it not "crack the engine block"?
This may seem like foolish conjecture, but it does point up the wisdom of keeping Muslim militants from ever gaining access to nuclear weapons and delivery systems. I do not believe such a scenario would be beyond them. As desperate as they are to usher in a worldwide Muslim apocalypse, they might well relish the opportunity.