SF Geek mode on:
Whoa, must be one *very* small amount of antimatter in the bomb and one *very* resiliant tunnel.
In the interest of credibility, never invoke antimatter in a story unless you are quite sure of the energy release even a miniscule amount of the stuff would cause.
Unlike fission or fusion, all of the anitmatter would always be consumed in the explosion, simply because it cannot survive contact with normal matter.
