Unfortunately, they hide underground like gophers and vermin. Flattening Gaza won't get them all. Fuel/air bombs would suffocate them, suck all the oxygen from the tunnels. Right now I'm dealing with a gopher infestation in my yards. As much as I'd like to use explosives on them, the international community (my neighbors) won't allow that. So it's down to killing them one by one. Similar to what Israel will need to do, get them one by one until the infestation subsides.