Jdams won't work to get into deep tunnels, because the fragmentary material they shoot out stops at the first twist the tunnel takes. To avoid them, combatants just need to go deeper into the tunnel. Bombs designed specifically to penetrate underground pose similar problems. Though effective when targeting individual below-ground targets, they struggle with crippling long, winding networks. That's where a massive concussive bomb has the advantage: Its blast can turn corners, and push all the way to the furthest reaches of a cave.

"We made Moab for this kind of target," says Cancian. "My guess is that we just didn't know where these tunnels were before."

Deploying the Moab in nearly any other situation also presents some insurmountable drawbacks. Its sheer size means only certain aircraft can deploy it. Plus its large blast range makes it inefficient for targeted mission. But by far the biggest impediment to using it more often is the risk to civilian life.

"These caves I'm assuming are out in the mountains, in a very uninhabited spot, so you're not as worried about civilians. But to drop something like this in Mosul, you'd level half the city," says Cancian. That kind of fallout likely explains why the Moab sat out the heaviest fighting of the Iraq war.